We have outfits here that advertise 'no overtime charges ever'. Thing is, if you call them at 2am they have their answer service read a script that says 'we are experiencing a high level of calls tonight, and may not be able to get to you for another 6 hours!'
They also charge a higher diagnostic than most, or want you to join their maintenance club before they will come out.
I believe most customers are reasonable about maybe taking a couple of hours off work to meet you during 'regular' hours. If you don't charge a little extra, what is there to stop them calling you when they get home from work or just working you into their schedule?
Let's face it, a little old lady or family with young kids and no heat is an emergency, a lot of other calls are not.
I had a customer like that, a restaurant that call me on weekends and night only. I had to "fire" them. I have never charged overtime since 1972, I have never offered 24/7 either. But I have always got my customers out of a jam.