Tipping can spoil it for other punters unfortunately.
The difference with other tipping opportunities is precisely what?
I think I've tipped a couple of times, but only a fiver (I think 10 once when I got an unexpected CIM and she was a hoot in other ways too, it was a while ago) and I'm usually a cheapskate 30 min milf-hunter anyway.
I felt the lady (uh!) earned it both times, happy to encourage her because it was beyond VFM in both cases.
I've never felt it was expected. They seemed delighted but I still think I got the better of the deal deal, it wasn't loadsa money - just a token and I think both parties thought that.