Depending on what you do with 3d modeling, it's best to keep your geometry in quads (4-sided polys) for the sake of deformations. Most game platforms have no issues with triangles. 5+ sides are going to give you outright bizarre deformations when animated.

If it's going to be a perfectly flat surface that is not going to be animated or interacted with (dynamics), you can have 100+ sides for all anyone cares. :p