That's a very good point, many airlines want the jumpseater to be one of the last ones to board and unfortunately it means the overhead bins will be very stuffed already. Now, if you know for sure that you'll be riding mainly on RJs where most carry-ons are side checked and loaded in the belly of the aircraft, I'd say go for the 26, it's always nice to have extra space.

A couple of times I saw jumpseaters leave their bags on the floor in the cockpit even though they had a seat in the back. I remember one time where I had a ticket and was already seated there was a jumper who simply could not find any space for his regular sized 22 inch bag, imagine if it was an oversized 26 inch bag?

A flight attendant and myself were trying to help him rearrange a few overhead bins to find some space for his bag but we just couldn't find any space (it was the Superduper 80). Finally the captain said to leave his bag in the cockpit which he did.

I look at jumpseating as a great privilege to all of us and try to stay out of the crew's way as much as I can, therefore in my opinion bringing a regular sized bag might help the boarding process some. However, if the 26 inch bag works for you - more power to you...