I've often seen one girl, then gone to another booking straight after, not having a full shower & only washing my dick in between. Any saliva they leave on my body will still be there.
People being people hygiene varies across the board, not all people will wash as thoroughly.
Also, I would say I'm not as enthusiastic by the second or third as I am in the first, I can see that being true for some girls too, like any job a the end of the day.
As pointed out, not everyone who thinks they're first is always first, however, without a doubt sometimes they will be.
I'm not someone who's ever cared about that, however, I can see logical benefits to trying to be first, nothing to do with the sp's occupation, just the fact they're people.