Quote from ajna:
In my own experience I find that whenever I get my limit order filled at the mid price quickly after entering my order, invariably the price will remain better than my trade price for the rest of the day. Conversely, if I need to fight a bit to enter a position, the price I obtain ends up being a good one. This has led me to trade in a manner where my buy orders are a bit below the current mid, and my sells a bit higher. This way I get a price I am happy with, and typically get a price that seems "fair" to me as well. Obviously if you really need to make a trade immediately, then I'll take whatever I can get at that moment. But even then, limit orders are a must. Even on the larger index ETFs, spreads can widen, in which case you could give away nickels or even dimes away. I would rather go through the headache of fighting for my price in order to keep those nickels in my pocket.
ST