I played bluegrass for a number of years before getting more into old-time and Celtic music, and I don't remember the foot lift being used as a signal in bluegrass circles. But bluegrassers usually use those "shave and a haircut" type endings, and anyone who's paying attention will quickly recognize the initial notes of that sort of ending lick and know the end's coming up fast. Since old-time and Celtic players don't usually incorporate any sort of ending, but just "chop it off", it's more important that everyone knows ahead of time that the end's coming up.