I used to do this on my oval car and just weigh the left rear tire to set the crossweight, but on a off road or road course car it's too cumbersome to use just one. And if you only have one, how do you know it's accurate? Plus the scales are insanely cheap now compared to 15 years ago.

Usually, I have my scales set up and check wheel weights before and after every run to make sure they haven't changed, particularly if I got in a big crash. I write them down at the beginning of the race day, and if the weights have changed, something is bent or broken. Makes it easier to keep an eye on things if you're running multiple classes.