When people lie in the sun to tan they rarely stay completely still and so those areas do get to see the sun. They may not be as tanned as the rest but they sure as heck don't have such a defined line as a spray tan does.
Sorry but you may as well say only do half a job in all your treatments. Doing a thorough job is why people will go back to you, it's what will enhance your reputation and get you repeat business ... SO GO THAT EXTRA MILE AND GIVE YOUR CLIENTS WHAT THEY ARE PAYING FOR!
If you ask your client to move so you can tan all areas they will know exactly what you mean, so don't worry about having to find the words so as not offend. And do not ask the client to hold their ****s etc, it is the perfect way for them to get tangoed palms. Ask them first and then, with confidence, do the job for them. if you do your job properly they will not be embarassed.
Oh and just remember that we get one hell of a lot of clients on this site who may be put off having a spray tan with some of the rather personal wording on this thread. This is not a staff room, it is an open forum available for all the world to see so please don't forget it.