Slow cooker - start from 'cold' or boiled?

According to official Crock-Pot website when asked about refrigerating the insert with food overnight, then cooking the next day:

Yes, but it is very important to understand that starting to cook with cold stoneware and cold food will affect the ramp up time of the slow cooker. Always add sufficient time into the cook time to ensure food is safe, as well as tender. It is recommended to use an instant read thermometer when cooking with chilled stoneware and chilled food to ensure food temperatures reach well above 165°F and food becomes tender. Never place your stoneware (whether it has been refrigerated or is at room temperature) in a pre-heated slow cooker base.

So, starting with cold, cool or room temperature food will be okay.

Bringing it to the boil is standard practice and recommended by most manufacturers along with preheating (the latter isn't as necessary for metal inner pots as it is for crockery ones).

It's a good idea as the heating power is rather low, and it will take a long time to get up to temperature (as I found when using one for mulled wine, which isn't a hazard kept warm).

However your simmering for 10 minutes isn't usually necessary (kidney beans should be boiled for 10 minutes, or even 30 in some sources so they're an exception). The surface of items in the liquid has already reached almost boiling point by the time the dish comes to the boil, and the interior of each piece won't heat up appreciably faster on the stove than in the slow cooker. The interior temperature is much less of a concern anyway; think how long it takes a roast joint of meat to cook through.

There are plenty of foods that are suitable for just putting in and adding boiling liquid, without bringing the whole thing to the boil. These are generally low risk and low thermal mass, like porridge oats, then of course there are things that use no liquid like jacket potatoes. Slow roast meat (like a whole chicken) is generally nicer if browned before putting it in, but there isn't any liquid to bring to the boil - preheating is more important then.

I often assemble hot ingredients in mine using it as a reserving dish and giving the final stir, but we're talking about adding fried veg, then boiling stock, beans etc. (I can't remember the last time I cooked meat in mine, though I sometimes use meat stock). At all times the food in it is above the danger zone.