Well. Yes and no. If you're too high, you will extract some tannins that will put a husky bitterness flavor to your beer. Not a good thing. If you are too low, you won't extract as many sugars from the mash. Also, at 168F you denature the enzymes, but I've been led to believe by many experienced homebrewers not to sweat that too much, since boiling the wort will denature them anyhow.
Most homebrewers that are using a cooler for a mash tun, are adding there sparge water at higher temperatures and it's more common to miss low instead of high.