Not as important as you think. One of the key measurements on an antenna is “beam width,” which describes how far off of that “perfect aim” you can be without losing signal. Even an antenna with a relatively narrow beam width like this Televes DAT-790 LR Long-Range Amplified UHF TV Antenna with LTE Filter (149741) from Solid Signal can still cover a 27 degree swath of sky, meaning that it can be up to 13.5 degrees off axis in either direction and still get satisfying signal.
This doesn’t mean that you shouldn’t try to aim as best you can, but a TV antenna is expected to pull in signals from many different towers and unless you’re in a city where all the towers are in more or less the same place (like New York City or Los Angeles), you’re going to get good signal from some and bad signal from others. It’s going to be a compromise, and an antenna with a nice wide beam width is going to help. If you’re looking for something that really helps you get signals from virtually anywhere, you can try an omnidirectional antenna which doesn’t need aiming. These antennas are generally only good for relatively short range, so if you want to really get distant signals from a lot of directions you’ll need something like this Xtreme Signal HDTV 8 Bay Bowtie Outdoor TV Antenna 60 Mile VHF/UHF (HDB8X) from Solid Signal which can adjust to cover an almost 180 degree stretch of sky, and pick up UHF signals from almost 60 miles.
If you want the best possible result in all weather conditions, it’s going to take a little time and effort to fine-tune. Most of the time you’ll be able to get great results with almost every channel with just a little bit of time spent, but if you’re one of those people who really chases the best possible signal numbers, be prepared to work for them. On the other hand, you might realize that you really don’t need to spend all that time to actually get good reception that doesn’t break up…. which is really what we’re all looking for, right?