Honestly I only upgrade 20k and up items. No reason to waste 300 inferno/aqua going from 10mcs charm to a 15mcs charm. But that's just one man's opinion
If you hav resources upgrading everything is the best. But if you don’t only upgrade the equipment that gives more stats for you upgrade. During this event if you had to upgrading the chest armor yielded like 4mill of charm stats or more per upgrade vs the necklace which yielded like 500k. I forgot the rates but it should be 2/2 4/4 7/7 8/8 10/10 for aqua/inferno so even just leveling them up just a bit is the best if the stats are not yielding as much as a chest piece or some of the bigger item. Hope that helps!
For old equips its a no no, it cost a lot of aquas and infernos and has a little stats, for new equip yes its good,less aquas infernos more stats