I have a regular WL that I see, and it would be about 7 years now, and she's more or less the same as when I first saw her. I suppose if you compared only one woman, over a long period of working regularly, it's only natural for her to be not as tight. But to compare multiple women, and then try and label them as having worked as a prostitute or not, I don't think you'll be very accurate. I used to think the same as you, but I've met some extremely tight women over the years as a punter and no, it wasn't in their first few months.
What I have correlated though is the size of the girl. Most petite girls I've had have been tight. The bigger girls (not necessarily fat/chubby, but including some tall, skinny girls too) were generally not as tight.
I've never asked girls whether or not they did those kegel exercises though...