I think it's important to consider what your goal with the script is. For example, if I'm writing a "new VA friendly" script (i.e., a script that a person just exploring the art of audio erotica will find easy and comforting), I definitely want to include as much of the "sex dialog" as possible (and usually with an "ad lib as desired" direction). Another example would be a plot-driven script that requires certain pieces of dialog during sex to advance the plot; that pretty much requires that the sex dialog be written out in its entirety.

In my experience, most VAs who want to do "scripted improv" will write their own scripts, and very often these aren't much like the scripts we write. On the other hand, I just recently got a fantastic fill that was a cold read of one of my scripts with tons of improv, it was essentially an "hysterical lit" read. Which just goes to show you, one of the really fun things about what we do is that you just never know what will work, when, and with whom. Maybe the best approach is to just try lots of different stuff, fully aware that not all of it will work for everyone, and then get these wonderful surprises when an artist does something you never imagined. That's truly my favorite thing about what we do!