What is the ideal fridge temperature

The USDA says refrigeration temperature should be 40°F (4.4°C) or below. If food is in there at a higher temperature (such as the 43-45°F the question mentions), for longer than 2 hours, and they're saying the food isn't safe.

Keep in mind that when too cold, after a while parts of things freeze, which can damage items, or severely diminish their pleasant taste. This especially goes for produce such as lettuce or even tomatoes. If you want to store leftovers longer, consider freezing them.

Edit: corrected to 40°F or below

Optimum temperature range for perishable food storage is 3 to 5 °C (37 to 41 °F).

I've noticed that there are differences in temperature inside the fridge. I think the top is a bit more colder than the bottom, I don't really know why. I keep my lettuce and tomatoes in the bottom, otherwise they'll be a bit frozen (like zanlok said).

So things that'll go bad fast (e.g. meat) go in the top.

Another thing (which I learned on House M.D.): don't put your milk in the door. The temperature there is more fluctuating so the milk becomes sour more rapidly than if you'd put it inside the fridge.

Regardless of what target temperature you choose remember that most domestic fridges cannot maintain that temperature reliably over the day and cannot maintain it evenly throughout the fridge. This error is generally improved with a full fridge

The best way is to measure it is with and accurate and calibrated thermometer on the shelf's you wish to store critical stuff, not the temp setting dial built into the fridge

4°C is common as it preserves milk well, but vegetables can suffer this low

It would be great if fridges came with four or five compartments, each with their own temp setting :-)

Different countries have different food safety standards. I'm accustomed to 4°C, which is the North American standard. However, in Europe I see that eggs are sold unrefrigerated and cheese is often kept unrefrigerated in people's homes, so I think there's a cultural component to the tolerated level of risk and the expected consumption time frame.

It also depends on the food items you want to preserve. Meat and especially fish should be cooler and vegetables and fruits a little bit warmer. The 4ºC guideline mentioned is correct. Notice that most fridges are warmer at the top and colder below.

Fish should be close to 0ºC for two reasons. 1. The enzymes of the fish will keep working (even when frozen) and 2. The bacteria of fish will be less affected by lower temperatures. The reason being that fish are cold blooded animals that live in a cold environment.

So, there is no one ideal temp. but a range from 0ºC - 5ºC for food preservation.

On a side note, family of mine have their fridges at higher temperatures so beverages come out warmer than we are used to. So, there again, there is not one ideal temperature but whatever you (and your peers) are used to.