Just to snipe patio11's inevitable reply:
http://www.kalzumeus.com/2011/10/28/dont-call-yourself-a-pro...
How much value does a few minutes/hours/days of your time create for them? That's the only question that matters.
If you can consistently create a lot of value, it doesn't matter how much time you need to invest, you deserve to be paid for not only the time you've put forth, but also the potential time that you might need to spend fixing stuff if something goes horribly wrong.
"Oh hey, I can't get MySQL to restart." This 2 minute fix just became a half hour of debugging. Maybe you've discovered that the RAM is bad and the HDD is on its way out, now you have to plan and execute a server migration with a minimal loss of business. How many hours do you think that will take? (If you say less than 2, you are a superman; please teach me your ways.)
Clients don't care about these details. All they want is for a business problem to be fixed in a predictable amount of time for a predictable rate. These details are your problem.
Charging an hourly rate makes it their problem.
Even though it might not seem intuitive, you're doing them a favor by charging a day-rate and exercising your discretion whether or not you even invoice them for a quick fix. You're the domain specialist.