I haven’t done that yet, because of gold. In-game currency has become more rare if you don’t sell items since the last update, so I wouldn’t want to throw it out the window just like that. The differences between levelling up is only a 2% increase on all stats for your character iirc, so leveling character is less critical than leveling the gear you want. Some commons could have better/more desired effects than some rares/epics, so for those upgrading might be worthwhile.
A common lvl 16 is equally strong than a legendary lvl 4, epic lvl8 and rare lvl12, so the effect is the primary thing to look at imo (especially for the higher levels. In lower levels the power difference can be too big to ignore, but you get my point)
It could be worth it if you are to open up an interesting item at a higher level.
But that’s just my opinion, I believe there are people saying “upgrade all you can”