It's a matter of trust and comfort.

Some ladies will allow photos, others may let you after they have know you and ahve seen you several times.  Others just don't like having their photo taken.

Just remember don't go sharing them.

I have met many great SP's since I have started hobbying, and once in a while I think to myself - wouldnt it be great if I had taken a iphone photo of them to cherish our time together.  Nothing erotic, simply face shot or full body shot with whatever her outfit she had on.  I have never asked my SP's simply because I hate feeling rejected.  But, is the notion bad?  generally too much to ask for/unacceptable/prohibited?  My gut feeling is, if the SP is an agency girl, then its a definite no-no.  But I would imagine it wouldnt matter much for indy's, as long as they can trust me to not show anyone else.  it would obviously be bad if i go and show a friend or whoever and he knews the SP personally.....  

whats your opinion/rule?  would love to hear thoughts from both sides of the sport.

She has to trust you and your discretion. You have to deserve that trust. And be smart enough to destroy (or remotely lock up) the evidence. I took NICE photos during a Monday play date with an ATF. They were off my iPhone and in my hobby email account before I left the session. They will never make it to iPhoto or anywhere else in my civilian life.