I've not had a bad experience at my local dealer. I know some people have - it's a mixed bag really. Also they've never once tried to sell me anything extra...maybe it's because I don't give off a 'totally clueless about cars' vibe though as I have a reasonable knowledge of service items, costs, etc. I'm no expert, but not completely clueless.
If you bring your car in spotlessly clean they're unlikely to wash it, although no harm putting the sign up!
Enjoy the occasion - I'd say popping your cherry will be fairly mundane unfortunately!! Maybe you could try to spice up the atmosphere by wearing some sort of fantasy/roleplay outfit??