Have only done this when backing first CP, example was So You Think the year he ran in the Melb Cup, iirc I backed him at around $8's before his Cox Plate win & then layed off at around $4's on MC day for a very nice collect regardless of the outcome.

But of course if he had been scratched at any point I do my bet cold & seeing it was the largest single bet I have had on a nag, was a restless couple of weeks let me tell you.