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It depends a bit on context.
I better expand on this a bit.
I don't think there's one way of doing it, in informal situations a lot of players may start a tune tentatively, finding their feet after a few bars. This is not done for effect but has more to do with, as I said, finding your feet, remembering the tune, getting your brain in gear, that sort of thing.
Playing for dancers often there's the eight bar stretch before the dancers come in but you're watching the dancers all the time too and may adjust speed a bit if and when they require more speed. Again, not something done for effect. Things may be different if you're in a ceiliband where the drummer (or the pianoplayer) sets the tempo by 'counting in' (I was listening to a clip yesterday of an old band where a drummer gave four knocks for starters and everybody came in at a completely different tempo than the one set up, which always has a bit of a comical effect). There's ofcourse the old knock knock joke about the ceiliband, where after the knock knock they all come in at the same time, based on that practice.
I can't really think of situations in traditional setting where it's OK to crank up the tempo for effect, it would be considered in very poor taste.