mine is at the very back of the dropouts. It makes it easier to take off wheel and i have good chain tension. It really sketched me out at first b/c I thought my wheel was going to fall out of the dropouts or something but 2 good mechanics told me it was fine so I kept it that way. They said that as long as the axle bolt is completely covered by the dropouts that it would be fine.
If you put your axle too far forward in the dropouts then it's tough to get the chain off in order to remove the wheel and change the tire.