At weekend autocorss and track events, it's common to see the top drivers recording their tire temps right after a run. They measure the tire temperature of each tire by placing the pyrometer towards the inside of the tire, at the middle and the outside. As they compare their recorded information and their lap times, they will adjust the tire pressure. Of course, these are people with highly developed driving skills, who are already consistently holding the car at its top speed through every turn. They can turn a minute set-up adjustment into an improvement of a a few hundreds of a second. (I'm not one of them. I'm not even thinking of buying a pyrometer.)
..... if you can't pick a coin up off the road on the tread, it's not hot enough.
If you glue the coin to the asphalt with rubber instead of picking it up, it's too hot....
Street use wise, prolly the most common non-instrument requiring method for rear tires is the burnout test:
Touch 'em off then go look.
If the patch is as wide as the tread, you're prolly close enough to start.
If it's narrower, you're over inflated.
If it's got a not as dark section in the middle, you're under inflated.
Piddle up or down a pound or three from there to get small grip changes.
Fronts are tricky as the load they get is definitely tied to how you put it into a corner, the radius of the corner, line, suspension geometry, etc., so unless you're ultra consistant and going through the exact same corner(s) all the time, pyro readings are a little pointless.
Better to adjust that by chalking the sidewalls and seeing how much you can go either way to get a nice even pattern around the edge.
Pyrometers are great if you're at the championship race and are competing for boocoodollahs and need every advantage you can get, but for general street driving (spirited or otherwise) it's pretty high on the list of overkill. (especially if you're not going to log track/road surface temperatures and humidity levels as cross reference so you know what time of day or under what cloud to add or remove a 1/2 pound of air to keep the grip level about the same)