Q: So is it acceptable protocol to ask a prospective agent who has expressed interest in signing you if you can talk to his/her other clients? I've been wondering this in the abstract for a while and you seem to suggest it's okay!
A: Heck yes it's ok. In fact, if any agent suggests otherwise, head for the hills while thanking your lucky stars for avoiding that little disaster.
When a prospective client is poised to embark on the SS FinePrint I tell them to contact any client of mine they want. No cherry picking, no "here's the list." I hope my clients are happy with me the day people email, but if they're not, that's my problem!