So, if I am EV training a Litwick, do I need to switch out to another Pokemon to gain less exp from the battle, so that I can "stuff" EVs into a level-up? Is this neccesary?
I am not sure what you mean? You do not need to get all your evs in 1 level. As a matter of fact it is better to gain level ups in B/W while EV training so that way it will save time on leveling up to do free battles on Wi-Fi. It will take longer to switch to another Pokemon in EV training because it will split the evs. If you want to do this you could give the Pokemon an Exp share if you want to cut the exp.