It depends. I guess this is a subjective matter which highly relies on the situation of a place where you plan to adopt bitcoin as payment method. If you are going to use accept bitcoin in exchange for products or services, then your rate shouldn't be fixed because it could cost you your supposedly profit if the value of btc is low.

Processing time and fee could be a problem if the network you'll use is usually crowded. In addition, the price fluctuations could be a factor too since most people tend to panic over sudden drops, hence, not really a good time to pay with crypto resulting with probably lower transactions if you're planning to solely set up a payment method with it. If you'll use it as an alternative, perhaps it could work out depending on the network you'll use and strategy you're going to make.