I have a quick question. I did a movie poster for a client a few months back. I was paid for the work, and the final poster was sent. They now want a few edits to the credits. Would you charge for this?

Unless you have already built in changes into your original charge, you should charge for the edits.

If I had the plumber put in a sink, then later ask him to change the faucet, should he do it for free?

It would depend on the nature of edits, if its a few bits of text and its going to take you 5-10 minutes, you have to weight up the ramifications of approaching the client for more money and how much the job is actually worth to you.
You say they want to change the credits slightly, I'm assuming these are all text? and if you have the original psds it shouldnt be a big job. If thats the case, to be honest, I wouldnt bother.

Bill for your time if it's an hour. Anything under an hour I don't bother.

I just finished edits to a file that was approved 3 months ago. At first the edits were small and took little time. In the end, I had spent 4+ hours on the project. You bet I'm charging. Met with the CD this morning and handed her a disc. She thanked me for the rush edits and said "Just send over a bill".

Thanks for the responses. I did charge and I am glad that I did. Like Ant said, a few quick edits has turned into several hours of work.