Is it bad to leave the crock pot on "warm" (not low) all day?

I suspect "warm" is meant to maintain temperature, rather than raise it. My crock pots are too ancient to have anything other than "high" and "low", so I can't assert any real authority. However, if you reach 145F within the first hour at the highest setting, then keep it at "warm", and test the temperature after about 30 minutes with an instant-read thermometer and it stays around 140-160F, you'll probably be fine. Personally, I'd test the temperature first by cooking water.

If the temperature stays above 140F at low, the worst risk you'll have is overcooking. Beans and vegetables like carrots and celery can overcook fairly easily in a crock pot, but higher collagen meats meant for stews tend to be fine when cooked for extended periods. Most crock pot recipes for stews and soups usually hold fine when at low for a full workday, although that's presuming a somewhat 70s-era soup aesthetic, which is probable for a crockpot recipe.

However, I would be inclined to attempt the recipe using the low setting rather than reducing it to warm, if you're not going to test the temperature first. If it turns out to be overcooked, you can always puree the ingredients with a blender...

I also doubt that switching to "warm" would be dramatically less likely to overcook the food than "low", unless it holds at a pretty stable 140F, and low ends up somewhere around 160F.

will it be a bacteria minefield or will it be warm enough to keep the food safe from bacteria?

I can't really stress how important it is to have and use thermometers no matter your experience level. And especially if you are unsure if what you are cooking is safe. I prefer a non electric (less parts, requires no batteries, unlikely to break) meat thermometer.

According to one of the Rival Crock-Pot manuals, it should not be left on all day. I have no intuition as to why though, other than they offer this advice so consumers don't expect it to bring all foods to a safe temperature.

HOW TO USE YOUR SLOW COOKER

1. Place the stoneware into the heating base, add your ingredients to the stoneware, and cover with the lid .
2. Plug in your Crock-Pot® slow cooker and select the temperature setting from the three setting options. Low is recommended for slow "all-day" cooking. One hour on High is equivalent to approximately 1½ to 2 hours on Low. Refer to your specific recipe for more precise cook times. Warm is ONLY for keeping already cooked food at the perfect serving temperature until you are ready to eat. DO NOT cook on the Warm setting. NOTE: We do not recommend using the Warm setting for more than 4 hours.
3. When cooking is done, unplug your slow cooker and allow it to cool before cleaning.

For more manuals (and recipes and such): Crock-Pot.com Customer Service

Last night I accidentally left dinner on "warm" overnight and when I woke up it was at 147F (still with a fair amount of moisture). That was with a 6.5qt "SmartPot" model. Mushy but safe, IMHO.

will it be a bacteria minefield or will it be warm enough to keep the food safe from bacteria

The FSIS (food safety inspection service) has released time-temperature graphs. At 130F you not only deactivate bacteria like salmonella, you have a 7 log10 reduction in bacteria after 131 minutes (chicken), which is their "safe" limit.

They don't really advertise that you don't have to cook to 165F if you hold the temperature over a period of time. The USDA public temperatures are the instantaneous meat temperatures.

@JasonTrue's reply is the answer. Below emphasis is mine:

However, if you reach 145F within the first hour at the highest setting, then keep it at "warm", and test the temperature after about 30 minutes with an instant-read thermometer and it stays around 140-160F, you'll probably be fine.

That first hour is key. You want to get food temp raised out of the low temp danger range as quickly as possible. So high heat for an hour, then low or warm heat for the day.

You might notice many crock pot recipes follow this model.

If you depend on warm to raise the temp, your dish will very slowly raise in temp. That's a risk.

My routine:

• wake up, go straight to kitchen and start the dish on high
• get ready for work (about an hour)
• put the pot on low (or warm), then go to work

here is the latest that I just got from a live chat with CrockPot: Ashly: On your Crock-Pot, both the low and high settings will reach a maximum temperature of approximately 215 degrees Fahrenheit, but they cycle differently in that the low setting cycles off of that temperature more frequently. Therefore, the High setting will still cook the food in a shorter period of time than the Low setting will. On the warm setting, your Crock-Pot will be between 165 degrees and 175 degrees Fahrenheit. Please remember that these are all approximate temperatures and they can vary depending on what you are cooking. The Crock-Pot can also boil on High and Low. We do recommend that you always fill your slow cooker at least 2/3 - 3/4 of the way full, including at least one cup of liquid or sauce. You may have to adjust your cook times accordingly, especially if you are using older Crock-Pot recipes, or adapting regular recipes for Crock-Pot use. This will prevent the food from drying out and burning.

I have a Hamilton Beach Stay or Go. It boils on low setting. I don't know what the high setting is, except that it is hotter than low. It doesn't boil on warm, so warm is probably 145-160?F. The best way is to check the meat with an instant digital thermometer. Foods cooked to 180 should be safe for any kind of meat.

I have the Crock-Pot brand 7 quart manual crock pot. Looking on their website (crock-pot.com) I was able to find the following...

Both the low and high settings will reach a maximum temperature of approximately 215 degrees Fahrenheit, but they cycle differently in that the low setting cycles off of that temperature more frequently. Therefore, the high setting will still cook the food in a shorter period of time than the Low setting will. On the warm setting, your Crock-Pot™ slow cooker will be between 165 degrees and 175 degrees Fahrenheit. Please remember that these are all approximate temperatures and they can vary depending on what you are cooking.

So, if the safe temp for food is between 140 and 165 (depending on meat) then this should keep your food warm enough not to grow any bacteria and probably cook it if left on long enough.