Yeah but pt is still pt, and all of it is a bastard when not constricted. The only time I would use pt for a pergola is if its getting covered in vines and even then it gets painted. Maybe its me but I would say yes, 18 ft a 2x10 will sag.I would say that yes you will have a sag. Whether it will be enough that it will be noticable, I can't say.
I'm sure that our PT is different, but I would never even considerate it for a pergola. It could get ugly fast.