Someone may argue with me, but I say, what does it matter . . . . if it makes the model look better use it. But there are those who will say that triangles are BAD . . . extra faces = extra render time, more calculations. If you have a fast computer it should not make that big of a difference in my opinion. I think trianglular faces look better on curved surfaces, but there are no good reasons I can think of to use them on flat surfaces that I know of other than **maybe** texturing.