It's generally a bad deal to use miles for gift cards and things.

Since you have a nice stockpile of Chase UR, leave your United miles alone. You might want to transfer some UR to United eventually for an award flight. They still have some great values despite the recent devaluation.

Leave your US Airways miles alone as well. Those will in all likelihood become AA miles.

As for JetBlue, a short flight from LA to SF will cost you 5,000 points. If you ever need a short flight somewhere, transfer some AMEX MR to JetBlue to cover the cost.

Don't transfer the AMEX MR until you're ready to book a flight on JetBlue. And only transfer the minimum you need to. There are some better uses of AMEX MR than redeeming on JetBlue.