Quote from evo34:
Opens are not ideal for trading, but if your strategy depends on it, you can certainly try. To do so, you need to specify that the order is "on-open" and that it is to be routed to the primary exchange. Places like Yahoo report the "consolidated" open price, which is almost always the same as the opening Nasdaq auction price (for Nasdaq stocks), but is almost never the same as the NYSE open price (for NYSE stocks). The reason is that most NYSE stocks will trade very small on some ECN right at 9:30, but the NYSE specialist will not open the stock until 9:31-9:35 in many cases. So the "open" you see on Yahoo is really just the first 100 shares that traded anywhere after 9:30. The Nasdaq opening auction occurs right at 9:30, so it is usually the same as the consolidated published open price.
SO, bottom line is that with Nasdaq stocks, you can count on your backtested open price data at least being accurate (though will not include any possible price impact of your order of course). For NYSE stocks, it going to have a lot of error in it, which may or may not make the strategy invalid. You can grab actual NYSE open prices from Bloomberg, but nowhere that I know of for free.
As for market impact, you need to do a study of how many shares typically trade on the primary open in a given stock, and plan to be no more than x% of that volume. x is difficult to value, as you cannot easily empirically test market impact, so you will have to make an assumption that if I am more than x% of the open volume, I am going to move the price too much. Be careful with ETFs, esp. leveraged ones. They tend to trade less than you would think on the open (small pct. of overall daily volume, compared to most stocks).