Rule #1: The Captain's Is Always Right.
Exception To Rule #1: When you are absolutely certain the Captain is wrong, then, you should refer to rule #1.
It is impossible to tell whether disregarding the Captain's orders would have produced a better or worse result in any given situation. While diving without a tag line might have produced less problems on a subsequent dive, there is no way to tell what would have happened had the tag line not been used on the one dive. Unless a tag line is NEVER justified, then only the Captain has the right to decide if it is justified in the particular situation. Who are YOU to substitute your opinion for the judgment of the Captain?
Someone commented about 99% versus 1%. Even if it was 50/50, who are YOU to say which 50 it is? And, what if one decides to disregard the Captain and to do something his or her own way, and he, she or someone else is injured in the process? That person had better hope the injury was fatal and that he or she was the victim ... otherwise there will be the devil to pay.
Unless the situation demands immediate and decisive action, it is ok to ask the Captain if he or she would like your opinion. It might even be ok to offer the opinion without an invitation. But, unless the Captain's directives are followed, then it is a free-for-all.
NOTE: The Captain may delegate responsibility for dive operations to the Dive Master, in which case replace "Captain" with "Dive Master" in all of the above.