Here’s my personal opinion on 5* leveling:
If you have no champed 4*s:
Do not level your 5*s unless you have OML and Phoenix or GG. In that case, I think you want at least 6 covers for OML including at least one of each color and 8 covers for Phoenix (skewed to purple/red) or GG. And then I would level both to OML’s cover cap.
Less than that and I think you are setting your MMR up for facing teams that are much tougher than you want to face. And in PVE, Heroics will become even more impossible.
If you have a top tier 4* champed:
You can level OML, but I would hold off on the others unless it’s an extremely well covered Phoenix (or maybe GG, I don’t know how good he is). That said, I would still hold off on OML until you have 6 covers, preferably with at least 1 of each cover. At least 1 yellow is a must have.
If you have multiple top tier 4*s champed:
Same as above, but leveling the others a little bit won’t hurt you as much. You could take them to 270-280 and not affect scaling or MMR at all, and probably higher because the likelihood of one of your 4*s being boosted isn’t as low. That said, I wouldn’t go much higher than 300 for anyone other than OML unless you have 8 covers (10 for SS or probably Spidey).
If you have a single or multiple non-top tier 4*s only:
Why would you have done that?! Submit a ticket and beg to get your precious ISO back.
FYI: I didn’t level my OML until I had 6 covers (3/1/1) and I don’t regret leveling him at that point. I had HB, XFDP, Thing, 4cyc, JG, and Carnage champed. I currently have OML at 375 (5/2/1), same champed 4s and I haven’t leveled my SS (2/3/3/), Phoenix (2/0/3), Spidey (1/0/1) or GG (0/0/1). And looking back, I have no remorse in waiting that long to level up OML. Honestly, I didn’t find him more powerful than my 4s until he got to 360 (I think about equal at 345).