as far as i know all it would effect are it's base stats at the lvl it is currently at, so arcanine would be stronger than growlith at lvl 10 but at lvl 100 arcanine would be the same as if you had stoned it at a different lvl.
it does effect moves you can learn though. some pokemon lose access to moves from earlier evolution forms so make sure you don't miss out on something you may want.
