If I'm trying to get a Dr to sign with me and he is hesitant, I offer to do old AR at the same rate I charge for new billing. I make it clear that I don't normally do old clean up for the same rate, but I will for him/her. It is worth it to me to do the old stuff at a loss, but to get the account.
If I meet with a Dr who is signing but also wants me to do old AR, then I usually charge a higher percent for the old stuff. Usually just a couple percents higher than my normal fee. It really all depends on what the 'old stuff' is. If it's going to be cake, then no problem. But if it's a mess, higher rate.
Michele