The ability for me to be able to shower at the appointment is one of the big deciding factors for whether or not I will see someone. I have no problem asking because like you, I do tend to get dirty at work without the ability to shower before a rendezvous. If it is not available or permitted, I simply move on and thank them for their time. Amazingly enough, it happens alot, particularly those who tour because they don't want to run out of towels, but the sheets are fine, surprisingly.
Their desire for the upscale and professional gentlemen actually has nothing to do with ones occupation, more with the way he carries and presents himself. To be honest, you probably make more than many of those that have office jobs and are considered upscale and professional. You just get your hands dirty doing it. I know I do so don't worry about that.