Depends on the folder. I had zero hesitation about standing while climbing with a Swift, for example. The real issue is that most folders are "one size fits all" and have minimal fit options.

However, if you can't get the fit you like on your own, I say get some help with the fit. Even if they can't get the TR onto the trainer I'm sure they can figure something out.

Also you don't want a situation where they spend half an hour figuring out a good fit for you, only to find that for some reason you can't achieve that on the TR. Definitely bring the bike.