Depending on the amounts, I like a "signing bonus" system over a straight "retro pay" based on pay/credit.

Given the SAME AMOUNT OF MONEY TO DIVIDE AMONG THE PILOTS:

With a signing bonus system you get a pro rata share of of the money and all FO's/CA's are rewarded evenly.

In a "retro pay" system, guys who were more willing to "help the company" and spend less time with their families and hobbies are REWARDED for helping prolong negotiations.

I'll take the signing bonus any day, b/c I don't do the company favors during negotiations. YMMV.