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.
If you are paying a merchant in bitcoin then you should be receiving a discount verses the exchange rate because the merchant has lower costs of accepting BTC verses accepting a credit card. The merchant also does not need to deal with credit card fraud (they ultimately pay for this).
If I had to guess I would say that the merchant is likely the one who is getting a bad deal and is probably paying much higher exchange rate then they should be paying.