Upcasting can be a double-edged sword whenever you decide to use it because on the one hand, you will be upcasting the 3d8 damage with 1d8 for every spell slot you cast it with above the first level. However, you also are burning a 2nd or 3rd level spell slot for the spell, and you do have a lot of other spells at that level that can be used. If you want to save your resources and use them tactically, then you should upcast whenever you are facing the big boss and you know what he is vulnerable to. 4d8 or 5d8 of damage, potentially doubled whenever you throw an orb of something your enemies are weak against, is extremely powerful especially with the range of the spell.