On liquid stocks market orders work fine. Obviously, if you pay very high fees for removing liquidity on ECNS, and it makes a difference to your p/l, than you might be better off executing your sales by offering stock or buys buy bidding stock first.
However, in fast markets its nice just to get into the trade (by using a market order or hitting the ask to buy, etc.). Typically, in a faster moving stock you can hit a market order when the momentum starts and then, when you feel the momentum slowing, you can offer out of a long position or bid out of a short.
My level II is set for Auto. Auto is configured for ARCA first for market orders, then my software sweeps the rest of the ECNS and market makers, and avoids some fees for my firm.
My trading style isn't based on adding/removing liquidity and I pay an all-in rate. I analyzed my trading and found out that I add liquidity on about 25% of my trades, and without wanting to deal with checking up the ECN rebate/fee accounting every month (would be millions of shares to account for), I use market orders frequently in the stocks I trade. Market orders remove liquidity.
In liquids stocks, NAS market orders are fine, especially for 100 to 500 shares, in my opinion. HOwever, talk to your tech support or trading support desk and make sure you have all the Nasdaq routing entitlements available to you, and also that you configure your trading software to use those availalbe routes. Take the time, when it's misadjusted the market order fills will be bad, but when adjusted well it works very well.