theres a sticky re: evolution and crystalling i think...
my practice has been to max proc level on lowest exp lvl mercs first...that way if u get a lvl70 fully crystalled, for example, u can evolve lower level with lvl70 and be fully proc ready....however, its harder to level exp with increasing rarity, ie epic3 (max lvl99) will take longer than rare2 (max level 70)...so its always better to max exp lvl (to 70) before evolving.
u dig?