I think you'll find that most of us test 2 hrs after eating for various reasons:-
1) After eating, our BG rises and returns to something approaching our standard 2 hrs later, depending on type of food eaten (low or high GI). The 2 hrs gives us a standard for comparison. If you wait 6 hrs, then your results are pretty random. In that time, you would expect your BG to continue to reduce, but it won't neccesarily. It may go down to a fasting level, or it may go even lower according to exercise levels and as a result dump Glycogen from the liver to raise your BG to match your energy requirements. You could thus have an increased BG. My BG is about 6-8 2 hrs after eating according to food type, but after fasting 24 hrs (due to a hospital test requirement) my BG was 9!
2) By testing at 2 hrs, you can compare results with expectations that are set by doctors, diabetes organisations etc around the world to see how you're progressing. There are tables showing desired BG levels at the 2 hr stage.
3) using a constant 2 hr stage, and testing randomly after different meals at different times of the day, you can see how time-of-day affects you. For example, I can eat lots of carbs in the morning but my response gets worse as the day goes on. some people are better in the evenings, and can't have much at all for breakfast
Hope this helps and good luck. I'm on dietary control only as well by the way