I agree that it’s unprofessional, but on occasion, conversation with a certain customer becomes a lot more friendly and open than usual. I think there’s a way to remain professional, and non-intrusively ask that if they need an opinion or someone to review the finished product when published, you’d be happy to help. I think that may even work in your favour with the right customer. Just an idea, but I agree with djgodknows in that it will be inappropriate in almost all situations.
Note: I have actually had customers come back to me asking for opinions on what they’ve done with their finished work, and on one occasion, asking me to review/read her blog and see if she felt I could imagine any more graphics designed anywhere. That being said, I’m a graphic designer so it may be a totally different ballpark and I’m not sure if the same sorts of conversation ensues with other services. I’d recommend avoiding it to be safe