Here's what I think... pH strips give you a range of the pH (although I am sure you could by some that are more sensitive than others) and the zap test also tells us the presence of lye in our soap - in essence a range. So the zap test gives us the same information but at a much cheaper cost.
If we wanted to be sure, then we should dissolve a portion of each batch in distilled water and use a digital pH monitor, but again it would not tell us if there is a small piece of lye hidden somewhere else in that soap. Maybe that is why we cut the loafs into bars?
I used a digital pH monitor when I worked in pediatrics - they are fun to use, but I can think of other things to spend my 50-100 dollars on.
So I check my bars closely and do random zap tests on several of them. I have stopped licking them and now use a wet finger run across different areas. My batches are small... 8-15 bars which makes the process easy to do. The only time I have gotten a zap is when I tested fresh made batter (for education purposes
) and when I checked a non-gelled batch less than 26 hours old.