100% this. Sure, you CAN negotiate with the roadside vendors. But please, don't.
On the other hand, did you know you CAN negotiate the room rates at 5-star hotel chains? You definitely should :)
The one situation where I have had a lot of luck "negotiating" with 5-star hotel chains is when I'm already staying there and need to extend. In this case walking up to the front desk is more easier and far more likely to get you a good rate then enduring the hell that is trying to change your reservation online (Marriott, I'm looking at you).
This, just booked an hotel in NYC and on Booking.com was like 20% cheaper than the offer on the hotel website itself (where you had to pay in advance, while in Booking.com you would just pay at the hotel!!). I guess I could negotiate the full hotel price with the hotel, they are adding a ludicrous margin, but not with Booking.com (even if they are adding their good margin anyway).