I try to use my soil thermometer every month to take readings. It's important to realize that your soil is going to be different temperatures at different depths so you have to take it at the same depth each time.
Another good tip is that you might have micro climates on your property and this is a good way of finding them. These are areas that are generally warmer or cooler than the surrounding land.
If you do have a small patch of warmer or cooler soil, you can have a wider range of plants which is always nice. This is where, for example, you can plant some succulents in a temperate zone, or some apples in the semi-tropics.
Your soil temperature thermometer is the best way to find these little spots.