Heat from the pit or from players?
Hitting soft18 versus high cards shouldn't give you any heat as it's perfectly normal BS-play, let alone hitting soft17 against any upcard. However there are a lot of ploppies who think you shouldn't hit soft17 against a bad upcard (Don't take his bustcard!) and you might get heat from them but this shouldn't matter to you.
As far as hitting hard17 is concerned, I must admit that I have never done that.The index versus the ace is so highly negative that I'm either off the table or in with a minimum bet when such play would be called for. So it doesn't really add to your EV, at least nothing that would justify annoying the other players and puzzling the pit.
Francis Salmon