That is a complicated question from an owner/manager perspective. The standard has been: Time and a half to the customer for calls after some predetermined hour in the evening and time and a half to the customer on weekends. The reason being, that you have to pay the employee overtime rates because he is over 40 hours.
Having said that, the economy has pushed things a bit. At current, we set our Emergency Call rate to cover employee overtime. An Emergency Call is after 5 PM through the week, after 4 PM on Saturday, and all day Sunday/Holiday. We try to be mindful of the cost that puts on the customer, but if you aren't covering your expenses, then you won't be in business very long.
As mentioned, the market will vary depending on where you live and what the competition is doing. Many companies need to charge more because they are paying their employees more for the overtime. As a self employed guy, especially just starting out you have more flexibility in what you charge. No overtime charges can possibly give you an edge over the competition and help get your business going.