certainly if you can afford the 20% margin hit. Your regular customers could certainly get offended and go elsewhere though.
I guess that's possible. Is 20% discount too much? I want to put a nice juicy discount so that people will go to the effort of getting BTC in order to save £20-£40. If more btc accepting businesses in London did this I think there would be more of a buzz and the reason for getting BTC would be the discounts.
In another thread it was said that btc users are generally young and male. What the bitcoin economy needs is more women. From having had a few girlfriends in the past I know that nothing is more attractive to them than a tag with "20% off" written on it.