The highest chance of that happening is with a HG(hotel girl). It usually doesn't happen often though. With walkups its almost always the same girl, and if it isn't, or even if it is but you just don't like the way she looks you can just smile, say "thank you!" and walk away. Most understand and don't take it personally. Sometimes you get a girl with a bad attitude that slams the door on you, but usually they're quite nice.
Read this thread to answer some of you basic questions about the website and how all this works:
http://forum.sex141.com/eforum/v ... &extra=page%3D1