Is there a general rule regarding whether one should enter a limit order vs. a market order, and how far the limit order should be placed away from the market price? A limit buy order saves the difference between the ask and the limit price, but the limit order may never execute. So, I'm wondering if there is a general rule that says, e.g., by placing a limit buy order at the current bid, the order will execute 95% of the time, saving the spread, and more than making up for the opportunity cost associated with the 5% of trades that do not execute. Any thoughts on how to think about this would be appreciated.