If you want to use the toilet in a convenience store, gas station, or government building, it is considered good manners to ask permission: "Toire kashite kudasai" should suffice.
I had an interesting discussion with some colleagues in a government building about folks who came in and used the lavatory without so much as a "please" or "thank you". Needless to say, my colleagues were not impressed with my observation that we (the public servants) ought to be asking the tax payers for permission to use the facilities and not the other way around.