Is it better to blow out my used air filter or put in a brand new one before having a smog test on my '97 Tacoma? I've read somewhere that a new filter lets too much air through and negatively impacts the test, but I can't find where I read that.
A change in air flow will change your short term fuel mix ratio and likely make the engine run a little lean. That's actually better in most cases unless you are already running too lean in which case that will cause your NOX to increase. These engines typically run rich, so slightly lean during the test is preferred.
Your best course of action is to make sure you change the oil if needed, that the intake is clean, MAF is clean (provides correct air flow reading) and that your plugs are in good condition so you have a proper mix in the cylinders. Be sure to put a bunch of miles on the truck if you clean things so the computer can readjust if needed.