Your bid doesn't dictate your place on line, your order number does. When its your turn in line, if your bid is at or above the ask you should get filled. If you bid 20 and your turn is up and the ask is 17, you are still paying 20, along with trade fees, the spread between bid and ask is brokers profit, you don't think they will jump at the chance to sell to you for 20 what others buy at 17? I'm not suggesting anyone bid any certain way, just saying that you will pay what you bid regardless, as long as it's at or above the ask when your turn is up