I'm with Elisabeth in that I have never asked up front but I have only been ripped off one time in over 8 years! The worst part about THAT was it was a long time client wh apparently had fallen on hard times (NO pun intended!). I would have happily worked with him had he mentioned something. Oh well...definitely HIS loss!
I've been shorted a couple of times but not by a whole lot so I realize I have been very fortunate to have been blessed with wonderful clients.
Some leave an envelope before, some an envelope after, some cash on the table before, some on the table after. Gift bags, greeting cards, tied to a bottle of wine. It SO doesn't matter to me. Whatever makes you gentleman comfortable works for me.
It would be EXTREMELY difficult for me to do a Cash & Dash as I LIVE where I work..LOL!