>>500806>grid fashion made of only polygons
Here's a better one:
Mathematically speaking, subdivision algorithms and other deformation algorithms (animation) are designed to work with rectangular, flat quads.
In other words, any area of a model that requires large amounts of deformation, such as a corner on a subdivided piece, or an elbow on a human arm, should only use rectangular-shaped and flat quads. Anything else will cause unexpected results.
If you happen to be modeling an area that sees little deformation, and is more focused on the silhouette, such as a human forehead, then it's possible to use tris to merge strips of quads together in order to save polys. Not much deformation happens at the top of the forehead, no one's gonna notice if it deforms weirdly there.