Simple, quick answer to your question is, yes it is reliable.
Leather thickness as noted on the leather is generally a range number ie. 2/3 oz., 7/8oz etc. One ounce weight designation is 1/64 inch of thickness. Generally, the slight variations encountered in weight (thickness) differences within a single side of leather are of no consequence in the finished product. Larger differences from side to side of leather ie. 2/3 oz versus 7/8 oz. in different sides can make a big difference in the finished product.
Even if you carry a pocket leather thickness gauge, you would have to measure all the way around the side at intervals and still would not know for sure about the center portion you could not reach, and your findings would probably be very close to the range number written on the side of leather. So... within a single side of leather it becomes almost a non issue unless it was grossly mis labeled. Your eye will soon be trained to pick up even subtle differences in thickness. Can't beat experience.
Paul