I'm not an expert, but I know that recently there have been problems when people are active in too many sites at once. You end up not knowing what offers you've done, what you've signed up for, and you can easily end up getting credit twice (or trying to), which can then result in NOT getting credit for either, and it could even result in getting accounts closed.
I personally focus on SR and do MP halfway...I shop through MP when SR doesn't have what I need (bought something from REI recently and only MP has it), and I do their 5 point clicks when those emails come out, but I do the vast majority on SR.
I just find it easier to pay close attention to what I'm doing by focusing on only those two.
So I personally think you're signed up with enough.
