Yes.

CRJ:
You get United and Delta no matter where you are based. If you're LAX, ORD, or PHX, you can add on American* benefits. If you're SEA or PDX based, you can add on Alaska benefits.

ERJ:
You only get United benefits no matter your base. If you're SEA or PDX you can add on Alaska. It hasn't been officially announced yet, but sounds like to get Delta benefits you'll have to be SEA based. (Maybe LAX too?)

*note that American benefits are useless to us. You may as well save yourself the $1.56 booking fee on MyIDTravel and list yourself as a jumpseater. I only have AA because my parents live in North Carolina and AA is useful to them out of CLT.