Just bought a Polaris Predator, and I have to place my Green sticker on it. They mention on the left side of the ATV somewhere on the plastic...but I was thinking that the most asthetic place would be on the rear, under the taillight, on the visible part of the air box. Has anyone in CA had problems if their green sticker wasn't exactly on the left side of the bike?

I haven't, and I have seen them put everywhere. Whether those people have been pulled over I don't know.
Personally I try to follow where they want the sticker because I don't want to be hassled.

I try to put them on the left rear corner, but if there isn't any place there I put it on the rear somewhere.