I always refer to my trailer as a 30' even though it's 35' hitch to bumper. The model is a 30RIPR. It makes it easier to find campsites and I've never had an issue fitting a 35' trailer into a spot marked for 30' RVs. I'll be testing that theory in March at Myakka River State Park here in FL. The only spot that opened up in the last few months was for 30' and I jumped on it.
If it was for something more specific or legal regarding the trailer, I would probably use the full length.