Hmm, really depends. I have never really been very interested in OP levels, so I haven't invested much time taking advantage of them. I have done a few when I've found other people who wanted to do them and I would tag along, so I currently have:
Axton, OP8 Maya, OP3 Sal, 72
Then I have:
Krieg, 58 Zer0, 31
I've been spending time playing one character at a time to get to 72, then moving on to the next. The OP levels come from other people, mostly, which is why my Sal is just a 72. I just see it as me needing to farm more weapons, without getting the bonus of more skills. I'll probably work on finishing those when I finish ranking each character to 72. I'm working on Krieg now and will finish Zer0 after that.