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What are you trying to make? If it's icing/frosting, then room temp ingredients blend more easily/smoother. Ditto for cheesecake batters (try it once with cold cream cheese and you'll learn the hard way, as I did).

Room temperature butter is for cakes and cookies because you need to cream the two together to give structure, and that's pretty tough to do with cold butter.

Room temperature egg whites whip up more easily, or so they say. But cold eggs are easier to separate.

I think you answered your own question PerkyMac ... in general, you want your ingredients at room temperature unless otherwise specified, because they mix together better. The notable exception is pastry, where cold rules.

You can generalize from there. Scones for example, aren't exactly a pastry but you want them flaky so ... cold butter. For cheesecake, you want to mix the cheese and eggs together into a nice smooth batter so your cake will be velvety smooth and lump free, so room temperature rules.

Just compare what you are making to some known entity and you can pretty much figure out what you need and why.

what i can remember from pastry school:
room temp butter, egg whites & yolks will create more volume faster when they whip...as well as making a more even mixture.
room temp butter, in addition, doesn't wear the motor of your mixer as much as cold.

ice cold butter & water in pastry works just the opposite. does not incorporate & therefore creates flaky pastry layers.

i can check harold mcgee if you want a more scientific answer.

Room Temp egg whites whip higher and more voluminous than cold. However, cold eggs separate much easier than room temp eggs.

Heavy Cream must be ice cold for maximum...whippage.

Cold butter worked into flour creates "pockets" of butter which remain intact, even after the dough is completed. Warm butter mixes too thoroughly with the flour. When the heat of the oven hits those pockets, steam is released. This steam kind of "separates" the dough, making it flaky.

Cream cheese absolutely must be room temp before starting to make cheesecake to facilitate mixing with other ingredients. I'd rather see someone nuke cream cheese at 30 second increments until it's pliable than to see someone try to beat cold cream cheese into submission. You wind up blowing all kinds of air into the cheesecake batter, causing the souffle effect and creating a deep crevasse in your cake. (Only cosmetically nasty - doesn't effect flavor.)

I must echo those advising cracking open anything by RLB or Shirley Corriher. Alton Brown is another good one to watch.

You've all been really helpful. One sin I've been guilty of is creaming room temp butter with sugar (for cookies) and then adding cold eggs. I have also whipped cold egg whites for meringue. It seemed to work just fine, but now I'm anxious to see the difference with room temp. I hope the question didn't seem too elementary - your replies cleared up some questions that have been niggling at the back of my mind. Thanks!

@PerkyMac; I don't know where to put all the crap I store in there!