Nothing is 100% with terriers, its all about the trust you have for your dog and knowing the dog and how it works, acts around earths etc etc.
Neil youve been at it a lot longer than me and what youre saying does make sense. But thats just the way Ive seen it done and the way I like to do it. When on big old places with plenty of holes I 'll leave the dog off and let them do theyre own thing but the majority of average earths I'll see if the dog marks on the lead etc. first.