LogFAQs > #970087372

Only certain types of characters really benefit from going to 90. Yae Miko will appreciate it, since she benefits from Dendro reactions which like level 90, as do any anemo characters who like to use Swirl. But you can prioritize other things first. Most other damage dealers are fine with stopping at 80, and many supports that aren't themselves contributing damage can stop at 60 or 70.

Upgrading weapons is much easier so you can go to 90, but if you're low on exp crystals just prioritize the DPS character weapons and supports can be stopped at 80. Leveling from 80-90 eats a lot of crystals.