I'll chime in on this.....
I work for a little, tiny, miniscule, nat sized 135 operator, yet have written reciprocals with both Alaska and Era Aviation. We on the other hand allow anyone to jumpseat who is 121 or 135, sometimes 91 (corporate).
I am amazed at the courtesy both the companies we have agreements with show me. They are true professionals, and it is funny how the pilots always ask about and admire the "bush" guys. I guess because most of them started that way themselves and they know the conditions in which we operate.
For example, I have not had the opportunity to jumpseat anywhere for the last six months because I am the only line pilot and am on duty M-F 0800-1900......It hurts but it's fun!