No. I continued to jumpseat out of uniform since 9/11 without a problem. I think the MEC's (ours was) are plugging this recommendation, but I never adhered to it, and I'm not aware that any carrier has mandated those traveling on JS passes be in uniform.
I would check on it. It would stink to get to the airport and not be able to go anywhere.
I have been jumpseating since 9/11 mostly on NW and DL. The sad fact is that nobody really knows what the official jumpseat rules are from day to day. I've had Delta captains try to tell me I'm not allowed, and I work for a Delta wholly-owned for crying out loud!
The bottom line is it's up to whoever is at the counter and who the captain is that day. If they want you in uniform, and you're not, you're not going anywhere regardless of what the "official" rule is.
And if you go on NW, be ready for a not-so-random search at the gate. Every time I j/s they search me. I finally asked the agent one day and she said it's NW policy to "randomly" search all jumpseaters. It gets old when you do it every day.
I believe the only airline that you have to wear your uniform is Delta. I personally hate having to lug and wear a uniform. You stick out like a sore thumb and the passengers always want to talk about flying. All I want to do is get from point A to point B without any hassles.