Do you have a minimum charge for small updates?
I frequently get emails from clients with small updates regardless whether they have a CMS or not. I try to keep track of all these small changes because in terms of time, it adds up! I keep files and invoice perhaps once a year if is just a few hours here and there but when time has passed I always feel silly for charging a full hour for something that perhaps took 10 minutes, but these small changes and edits can be quite stressful at the moment and disruptive to productivity.
If you count receiving the email, trying to understand what the client says, perhaps a follow up question, opening project files, loggin in to ftp or cms, emailing the client back to tell the change was done, then try to get back into whatever I was doing, we are no longer talking about 10 minutes...
Several years ago a prospective client I met with complained about a quote they had received from another consultant because their minimum charge for change requests was three hours. This stuck in my mind as a good idea, but not something I have implemented. I would be interested in knowing how others handle this.