I don't dick-slap unless they ask for it, or unless I know they're into it (previous conversations or whatever.) I pretty much see it as rude as hell, and while I can dom well and have done so upon occasion, it's just not a turn-on for me (either dick-slapping, or D/s.)
That said, if it gets a partner off, I'll generally at least consider it, so long as it isn't too far out of my comfort range, or just too plain dammed stupid.