I will also add that I just got done doing a competitive analysis of pricing in my area and found that none of my competitors are charging premiums for businesses. That was found from a cold call to about 7 different shops.
Maybe they just don't advertise it, like I don't. I actually don't recall a single business customer of mine asking for a quote beforehand. So, I just usually make it up at the end. Then I just usually charge it as a flat rate instead of breaking it down by the hour for them. No one has said anything or given it a second thought so far. But like I said, I'm not really charging business customers more for the type of work because like you said, if it is harder work you will be there longer, hence making more money. I do usually charge a higher rate for the "priority service" so to speak. Again, not advertised as such.
When a business customer calls you, and you go right over and fix it, they are usually so happy that they don't care what it costs. You really can't make a business customer wait, if you can help it, because they could likely be losing money for every minute the computer is down. I am also willing to work on the weekends and evenings for business customers and I don't for residential customers. Sometimes you just can't take down a computer during business hours. I don't charge an after hours rate to them either, so the extra charge in the commercial rates averages out over time to compensate that.
I also shouldn't say that I won't service residential customers after hours, I just haven't offered to for anyone yet. If a customer explicitly requests it, I am pretty likely to do, however, I will probably add a "truck roll" charge to it.
Again, I have thought about just adding a "priority" service or an "after hours service" additional charges, but I don't want the customers thinking they aren't important just because they don't opt for the priority service. However, I am not sure I see a negative image with "after hours" rates. I may consider doing that. Most service companies do.
EDIT: This thread has made me decide to revise my pricing structure. After reading my owns posts, I feel there is too much uncertainty in my onsite pricing. I am going to have to nail down a concrete on site rate system. I am kind of leaning toward a flat rate for onsite work. I don't think I'm going to do a business and commercial, though. I think the way to do it is offer a flat rate repair if I will be working on a single computer. More than one will be charged hourly. Will work on it.