A lot more often than I used to, but not as often as a lot of players, I'm sure.
I always ask myself what I'm going to gain by making war. Cities? Luxuries? Desirable geography? Resources? I don't go to war for the sake of going to war, but if I can benefit from it, I go ahead.
And kman, don't overlook early, ancient age wars. Often an ancient age war is a quicker, easier, and cheaper way to knock down a neighboring civ that you know you'll have to take on eventually. They don't have to be long and tedious. In the ancient era, AI's tend not to defend their cities too heavily, so a fairly modest stack of Swordsmen, Horsemen, Archers, or one of the early UU's can pick off 2 or 3 cities that will really give your early development a boost.
I find that when I'm next to one of the more agressive AI's (Germany and Zululand spring to mind

) an early war really helps "keep them in line" and often eliminates the need to deal with them once they have tanks (or, worse, Panzers.)