Are you talking about "eating to the meter"? It can be quite useful to see what foods raise your blood sugars quickly because even non diabetics get that and because the body reacts to a rush of insulin to lower them, you feel hungry quicker. However, I think if you just follow the rules for a low GI diet, you'll only be eating foods that won't spike your blood sugars anyway so you don't need to test your bloods. If you are pre-diabetic, it can be really helpful because whilst porridge oats are considered to be good, my blood sugars shoot up when I eat them.