The answer is absolutely! Some businesses think a deal is complete after payment receipt, but if this is your way of thinking be prepared for that to possibly be your last job for that client. Just as in the non-profit sector, business relationships have to be cultivated and nurtured if you want to build an ongoing working relationship. Plus, there is nothing to lose, just the gain of a loyal client.
If you still don't see where we're coming from try this example; you own start-up graphic design firm X. Firm X has completed 4 jobs for a client this year and they "love, love, love" your "stuff". Meanwhile, Firm Y is prospect hunting and takes your client out for lunch, while providing them with samples of jobs with the exact same quality as yours. Whether or not you've shown appreciation for their business or even tried to establish a relationship could make all the difference between your next job and the loss of a client.
You have to remember, the same clients you're completing jobs or consulting for are being solicited constantly by your competition. So, even if it's nothing more than an E-card take the time out to say "Thank You".