I'm trying to get a Hydreigon w/ 252 sp. atk. and 252 speed, but I don't know whether to start E.V. training now or until Deino evolves (to Zweilous, then Hydreigon, of course). Help is appreciated, and thank you in advance! :D
Yes, EVs effect EVERY Pokemon. If a pokemon evolves, it still keeps the EVs it gained, so feel free to EV train as a Deino!