I rarely play On-line cash games, and at a Casino I have never asked for it, but if the person I am playing against does ask for a "Run it Twice" option I do oblige them because I do believe in good sportsmanship. It's a little like asking for a Split of the prize money in a Tournament. If I am short on chips I would never ask the other players to split the prize money, I consider it an insult to them, but if I am asked as the Chip leader in a game I would consider it depending on stack sizes.
Once I was heads up against a player who waited until he only had 2 BBs left, he asked me to split, and I said "you are joking aren't you?" I'm calling with any 2 cards you shove all-in with at this point in the game.
I don't like being taken for a fool in a situation like that but if we were closer to 60%-40% or 70%-30% then I would consider a fair and/or equal split.