If you decline evolution when prompted to evolve your pokemon, it'll prompt you to evolve your pokemon every time it levels up after that point - Therefore, you'll still be able to evolve it later, even if you decline it initially.
Also, some pokemon are better off evolves later (such as Growlithe, who learns flamethrower at level 49, which is a move its later evo Arcanine can't learn). You can look up a pokemon and its evo on the PWO wiki, and compare their move sets, to see what moves the current evolution stage of your pokemon learns that its later evo form might not learn or learn later.
Even if you evolve early, you'll still be able to teach your pokemon the move on playerdex later, once it reaches the level its previously evolved form was required to learn it at; however, this costs pokemoney ($500 multiplied by your pokemon's level), so for some pokemon, it's more convienent to just evolve them after they learn a certain move that their later evo doesn't learn/learns a lot later.
