How to use assist XP for T4

[LumpyMilk]

So i was using my assist xp and putting it into melee (same with my archer), because its the cheapest to get thus fastest to lvl.  But some guy (i dont remember who) told me to just save the assist xp then use it to get gold for hero once i get lvl 52 from kills.  Anyway, i was just wondering if i should spend my xp right away, or save it (for both inf and archer)