The problem is, if you have a 72 now, you feed in 30 minutes and then test him 30 minutes after that and say he is at 95, you have no way of knowing if that 95 is because of the food he got an hour ago or because of how is body is processing the insulin.
If you don't feed and he goes to 95, you know that it is just the way his body is processing the insulin and it is most likely ok to shoot.
If it ends up being the food that you gave him an hour ago, he will most likely then drop soon after that a bit from the food spike, but then you are also adding the insulin into that as well, and he may drop further.
Keep in mind that Lantus takes a couple hours to take effect AFTER it is giving, the two hours during when the shot is given and the onset of the next dose, his body is using the insulin in his depot. This is why it is often suggested to get a +2 or in some cats, like my Skooter up to a +3, to see what that dose is going to do.....