Oh, you betcha. It works especially well before you close on the deal. I actually have fun doing it, usually goes something like this:

Me: "Oh, just one more thing. We need to add an addendum to the purchase contract that you will remove the sticker before I take the car."

Salesperson: "That sticker is on all of our cars, we can't remove it."

Me: "OK, thanks for your time." Remove butt from seat and prepare to exit the premises.

Salesperson: "Hang on, let me talk to my boss, we might be able to do that."

I drive away happy and dealer sticker free. I think dealers are getting smarter about this. My last two buys (300m and Crossfire) have had the ridiculous plastic license plate frame dealer advertising but sticker free. I get home, take off the crappy dealer frame and install the one that I want. My car is then free of garish dealer advertising.

In your case, since you've given up your sales leverage, I would just say you'll never set foot in there for service if they give you any crap. And recommend they switch to the cheesy plastic frame thing, informal studies have shown that a fair percentage of people actually leave those things on their cars.