I'm not sure I've met a red wine that actually got worse with an hour of decanting, and most get better. I say, plan on decanting everything. Take a sip before you do so you can a) learn how it changes, and b) make sure it's not corked.
Any young red is sure to get better with some air time, especially Cab, Merlot and Syrah/Shiraz. Pinot is the one variety where my rule may not apply--many don't seem to improve much with air. But like I said, I have never known a decent red to be worse after an hour of decanting, including Pinot. Even the old Bordeaux I've had from the 60's and 70's has evolved nicely with an hour of air time, and did not fall apart as some writers had led me to believe. Give big young Cali cabs two hours.