They should all be 18.8, but like copper says, there aren't many forgeries of these out there, and the ones that are fake, are obvious.. Honestly though, weighing them isn't really going to tell you if they're fake, just find an example of the same year on a dealer's site and compare the lettering and the general look of it. If you're literally just asking the weight lenience on a penny, then I'd go with +/- 0.1g and the diameter should be exactly the same on all, unless there's an obvious problem with the coin.