Any (consistent) data you collect can and will be useful, it's a matter of being able to determine how it relates to general kart logic as well as how it relates to other data you are collecting. You have to know how to interpret and use the information once you have it. That being said, if your tire program or your setup is way off base to begin with or you don't yet understand the physics of how a go kart operates, taking tire temps isn't going to magically land you in the winner's circle. Temps come into play *more* so when you start fine tuning your setup after you've already found a solid baseline for your program and driver. I definitely believe that monitoring tire temperatures CONSISTENTLY can be a useful AID in determining how the corners of the kart are working together. However, you also have to keep in mind that there are many factors that can contribute to changing tire temps, especially in dirt racing since we cut, surface, stagger, roll, prep, etc. Thickness, surface type, compound, sidewall height/stiffness, prep type/amount, etc. all have an affect on when/how a tire heats up. Generally: a tire that's doing more work will be hotter because it's going through more manipulation and seeing more friction on track. Just like in Mike's example where the LR was the hottest, that directly correlated with the information he got from his driver (a push). This suggests that for some reason the LR is in the track too much. That example alone should show you that tire temp data can in fact be useful when used right. Physics is Physics. Just because you don't understand it doesn't mean it's wrong.
Taking the temps as soon as you are able to and keeping the amount of time between rolling off the track and hitting them with the pyrometer as close to the same as possible each time is what matters. If you do it after 5 minutes the first time but then do it after 10 the next time, your data is then "useless". Be consistent and take notes. It's all relative. Championships are not won by doing what everyone else does. Experiment and figure out a program that works for YOU and YOUR DRIVER. If you want to take temps, do it, but do it right. And like Mike said, use an actual pyrometer... Please. Also, you should definitely look into learning how to read the surface of the tire after coming off the track, that will tell you *almost* everything you need to know as far as how the kart/tires are interacting with the track surface.
When it comes to karting, tires are like the whole entire user manual of how to go fast, but written in a new language. Learn the language and then reference the manual frequently, it won't lead you wrong.