As mentioned by others, the $20 is in lieu of paying any additional $.
I always fold the $20 and put it with partially under my CC (with the 20 clearly showing) and ask the clerk if there are any 'COMPLIMENTARY ungrades available'. Be sure to say COMPLIMENTARY.
In addition, your best success will often be with a middle aged male clerk. Sorry to sound sexist, but this is well documented in other LV forums.
FTR, I have had success at MBay, Ballys, and the Mirage.
hockeystl