If you do this, you'll want to be careful of a few different kinds of situations:
* the brain pickers. They are looking for an inexpensive teacher for anything and everything. While I have no problems teaching my clients the basics of troubleshooting HTML or CGI, or the basics of configuring their FTP client or installing their SSL cert, I don't want to become their sole tutor without some compensation. For one, it's a huge time drain. For another, if I've already invested a hefty chunk of time writing documentation, I'd appreciate it if a customer didn't ask too many questions that could easily be found in the knowledgebase. Many people ask a few questions that are written up, until they see how extensive said knowledgebase actually is. Some people go straight for the knowledgebase and support never hears from them. Some people...just seem to want a private tutor.
* the delegators. They will want, or even expect, you to do quite a lot for them - continuously. Make sure to give yourself and them clear definitions of what is provided as part of your standard support, and what goes above and beyond. For example, I offer business and personal hosting packages. The business packages are more resource-rich, and are priced higher because I expect that businesses will have more "please take care of this for me" types of requests - especially for things like installing SSL certs, parking domains atop subdomains, even configuring certain software packages. When someone paying less than USD$6 a month wants me to do everything for them but wipe their nose, my own nose gets a wee bit out of joint.
Dealing with the extremes: Think of graceful ways to get the folks at the extremes to either compensate you for additional time, or learn to shift for themselves a little bit more. You might want to have a predefined set of support limits so that when a client exceeds those limits you can tell them: "As mentioned here, that particular level of support will cost you {however-much} and will include {service-service-service}. Shall I continue?" Then, for those clients who only ask occasionally, you can just not charge that extra cost.