I've paid several times in bars and restaurants with bitcoin (I go there especially because of bitcoin) and every time I was ripped of.
Every time I paid a premium of 5 to 10% above FIAT price when I paid in bitcoin.
5% is a reasonable premium, but 10% is pushing it. It also depends on which exchange you base the price on. Even if you use the highest priced exchange, you can't really use a 1:1 ratio, because it cost money to wire fiat into an exchange or withdraw fiat out, and another small fee to actually make the exchange. These fees are factored in the premium.