Rezzed prims have slight rotation on the Z axis?

I'm relatively new to SL, but have taken up the prim building technique pretty fast because of it's similarity to Quark, Worldcraft/hammer, and other map editors. I've begun to notice lately that everytime I rez a prim from my inventory, it has a very slight rotation about the Z axis. in general its' not a big problem, but now that I'm working on some very fine detailed items, that slight rotation everytime I rez up a save point, or want to utilize a shape I have on a different object can tranlate into some noticable errors later on.