Often wondered the same thing. Have never asked the question however.
Was just never sure how it would be interpreted.

Like you see a massage girl and just gives you a hand job but you want to fuck her?  Never hurts to ask, shes a sex worker, its not like youre asking the baby sitter to blow you.   Most massage girls clearly delineate themselves from full service gals for some reason.  Might be a boy friend issue or--in my impression--  the girls believe they are holding themselves to a higher standard by not blowing or fucking you.  Keeps the allure alive by not offering everything you want.  Good luck, ask them whatever you want.  

Regulars

As a non FS provider , I'd have to say it's is possible out there I can't speak for everyone  

But each encounter is treated differently.  

As a demisexual ,meaning,  I feel sex is best , for me. When there is  great connection and respect.  

 I'll admittedly say I HAVE opened my menu for very few with out regret.  It takes alot of trust and understanding and the funds absolutely help.  

 
It's hard for me to enjoy and 100% relax with a stranger  
And it's hard for me to see into the future on whether or not some is just there for the chase or if they will continue to return knowing they may not have the exact same ending experience.  

 
I genuinely and generally care for everyone but  I've also learnt that I do also get a little attached , if not more , due to wearing my heart where my gspot is , lol,  but once I do open up , I thoroughly enjoy sharing a very indulging experience.  

I have seen some escorts that are new to the business have restrictions that slowly disappear over time.  Two that I can think of that I visited at least twice over the course of a year or so went from CBJ to BBBJ.  Which turned out to be a pleasant surprise.  
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I would say if they have specific restrictions mentioned on their websites or wherever, you have to assume that's the limit going in -- and avoid asking before meeting.  Once you are in the door and having fun would be the appropriate time to ask.  Timing is important due to legal jeopardy.

I’d rather know up from what the boundaries are. I hate to ask for something and get a no because it always diminishes the mood and can create discomfort for one or both. Also, when I spend my money I’m doing it for an experience I want. I know that’s selfish, but it would be different if it was a friendship or relationship.