the lug pattern of some wheels are labeled 5 - 115 and others 5 - 114.3 i am assuming this means a difference of only .7mm, so will a 114.3 wheel fit on a 115 car?

It would not be safe. The loading on each wheel stud would be uneven, since the lug nuts would not be perfectly centered on each hole. Wheel or stud breakage may result.

^^^ ditto...

actually, if the hub ring is the right size then each hole will be .3mm off, which is negligible. Everyone with FWD GM's uses 114.3 wheels and rarely if ever are there negative consequences, just make sure you get the right size hub ring.

I respectfully disagree. I think they're lucky, but the key is to get the acorn to seat in the chamfer. Tolerances here are usually less than .005" which is 0.05mm. Anymore than that and either the wheel or the lug is bending more than acceptable amounts.

http://www.winstonsarea51.com/bolt1.htm
http://www.mrt-wheels.com/boltpatternref.html