Depends on the project in question. Some don't aware enough, so really should be giving more per hour to bring them closer to the average. I'm talking about Rosseta and WCG as 2 of the big offenders for low credit return.

My strat is usually to pair up one of the high payers with something that pays poorly. On the one hand I wouldn't want to do nothing but math projects all the time, on the other hand, the credit return for some is so ridiculously low, they really could stand to give more credits per task. But by running them together, things fall more in the middle, and the higher paying projects gives one the headroom to run some of the over nerfed and ridiculously low paying projects, without the RAC taking a big nose dive. It can then sorta balance out, in the middle somewhere