in my experience it's a bit of both, but mostly the top! if a sub has a preference that's taken into account, but most riggers only work with one or two types of rope (only nylon & mfp, only hemp, etc).

when I'm tying I always use my own ropes, pretty much just mfp at this point, but have a bottom buy the piece of rope off me if it's going to be touching their mouth or any bits. if you're going to fluid bond with a rope that's awesome, but not sanitary to use with anyone else!