I never ever ask if they are married, have a girlfriend or kids, but I tend to view occupation as a safe topic and I do ask this as an ice breaker in conversation.
I agree that family, girlfriends or kids is a topic I would rather stay away from, it feels too personal (and a bit hypocritical given the situation!)
I confess I usually do ask about their jobs! And if they tell me a bunch of porkies, it doesn't really matter
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I find they are quite happy to answer and it makes easy relaxed conversation to break the ice. I've never considered that it might be interpreted as prying into how much money they can spend! That makes me feel a bit guilty now but that's really not the reason why I ask.
I usually bring the conversation around to the topic of work with 'so have you come straight from work/had a busy day/got the day off' or some such thing. I probably am being nosy because I do find it interesting! But it really helps me get a feel for the type of person and helps me connect with them, which I like. Not just during the booking but it also helps me remember who they are for future (especially since many guys give the same false names!) so I have a lot of numbers saved as 'Steve Does Shifts/Steve Surgeon/Steve Taxis/Steve Restaurant'
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Which means I remember them more easily because I have a clear memory of talking about their job, and for some regulars it means when I see them next I can ask 'so how is the restaurant business going?' which they seem to appreciate that I remember/care.
I once had a client compliment me in a review saying that I was warm and friendly and seemed genuinely interested in him and what he does, so I haven't ever regarded the topic as taboo.