From what I've read, paying an old chargeoff temporarily decreases your score because in a sense it dredges up an old dormant, but delinquent account. I think using an old credit card that has been inactive for a long time may have the same effect of temporarily lowering your score. But it makes sense that paying off your bills (and chargeoffs) will help in the long run. I say that because whenever i'm declined for a credit card they always say I got declined because I have a chargeoff. So, I assume that paying off the chargeoff, even if it lowers my score will be better when I seek credit.
In my case, I was going to let the charge off fall of my account, but after 4 years, I just settled it and they agreed to report it as "paid in full" even though I only paid 1/2. I'm just going to wait a couple months and see what happens to my credit. I also took out a secured loan at my credit union, and that will also decrease my score a little at first, but as I pay, it will increase my score.
In summary, it seems every sudden change (e.g. using inactive account, getting new credit, paying chargeoffs) decreases your score. But paying on time for months and years will inprove your score. Remember that you are in this for the long haul! years!