Nothing insulted me more than a bribe (yes, a bribe) handed to me before I had even opened my mouth to greet someone.
First of all, how dare you think I'm not going to provide excellent customer service unless you grease my palm.
Second, I'm not going to treat you better than anyone else simply because you chose to insult me with a bribe. Why should I treat you any better than the lovely couple in the other room? Because you gave me money at the beginning of the trip instead of the end? What an entitled attitude.
Third, giving someone money before they perform their service is not a guarantee they will provide excellent service. Hey, thanks for the $20. Now I can provide basic service for the rest of the trip because what are you going to do about it? Chase me down and say, "Hey, I gave you $20 to ensure I receive good service," and I'm going to inform you I'm not obligated to do any more than what's required of me, but thanks for the $20. This is why you always tip at the END of the trip. I don't even pre-tip delivery drivers. If they show up with a crushed pizza box and say they forgot my soda, it's not like I can take my $5 back.