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I believe this is a highly necessary step, but one which is not likely to get useful answers! Its primary purpose would be to show follow-through and interest in their opinions and satisfaction.
I'd of course pay attention to what is said, but it is not likely to be anything negative in most cases.
In addition, I'm beginning to create a newsletter of tips and useful bits that I'll be offering customers and former customers--probably via a new website. Anything to make them keep me in mind for their next projects seems to be worthwhile to me.
As for gifts, I would be reluctant to suggest any particular thing. Even in the various advertising specialties, the only ones that really interest me are things that can be *useful* for the recipient...and those things can be expensive. If you decide to send a gift of some sort (and in those circumstances I actually like Darren's idea because it is matched to the interests of the recipient), I'd make it as a response to such a satisfaction survey. Sort of a "Thanks for taking the time to respond to the customer satisfaction survey. I saw these and thought you might appreciate them!"
ALL OF WHICH SAID, I would *NOT* do it via email! That is too common today. Anything of this sort I would actually send in physical snail mail...as I usually would for invitations, birthday greetings, and the like. A "survey form" on paper that they return could be used in a variety of situations later to accompany any portfolio entries that might include work from the project.