All over the forums you see people saying don’t use n-gons or triangles as if doing so will trigger armageddon itself.
Seriously though, why shouldn’t you use them? Bad topo?
There’s nothing wrong with using tris or n-gons in certain situations. Games models almost exclusively use tris because it’s more difficult for game engines to render quads without slowing everything down and FPS is the name of the game. N-gons sometimes cause weird rendering artifacts, but if everything looks good when you render, go for it.
The main reason people say that is they create artifacts when using subdivision/subsurf (eg can’t subdivide a triangle cleanly like you can a quad). They also complicate various modeling tools because you can’t loop cut through them for example.
If they’re not actually hurting anything though, you don’t need to worry about them.
Thanks guys! 
Real interesting stuff.
i use tris all the time because i don’t work purely with blender, obj files which i import usually contain triangular geometry even where it could use quads. the problem with tris (and n-gons) is that they do “funny things” when one renders and they cause nasty very dark or very light triangular patches against the rest of your geometry. used carefully either can be fine but in the wrong place they cause very annoying trouble with how the render engine will interpret them. if you see these annoying blemishes you need to rework the bits of geometry around where the patch appears, otherwise don’t worry.
Some programs might not be able to deal with n-gons nor tris. I think PMG’s Messiah is once such program?