I've made had any problems with this.

One of my regulars probably goes to the loo each time we meet but also has a quick wash for me after.

I don't allow any client to use my bathroom till I've been paid. Like others have said some use loo or shower and then fuck off.
Money first.
Then bathroom second.

I do not like them trying to use the loo as soon as they enter. If they say just need to use the loo i say yes sure just pay first please.

I had 1 guy ages ago that went to go straight into the bathroom without saying anything, i said can you sort the fee please he actually had the nerve to say NO i won't i will do it later. I said No it needs to be now!! He still refused so i went into bathroom in front of him and said you need to leave right now i said it's obvious you are trying not to pay. He stormed off muttering something. There is no way he was going to pay anything. Either he would have had a wank and left or tried to get a free service without any payment.

Although there might be the odd genuine guy needing to use bathroom when they arrive but not worth letting them til they pay up.