while it might seem automatic that all things interpersonal would intersect with all things tactful, there are times when tact would soften a message too much, a situation is too much of an emergency to have time for tact, or conversely situations demanding extreme levels of tact. could be useful!
This feels a bit like an extension of should all things be polite? I don't see how [tact] can be the subject of a question. If an approach needs a particular degree of tact (whatever that means), it should be described in the body of the post. But I don't think we need a tag to say "be tactful"* as another way to say "I'm just trying to be polite here". It is simply endemic to this site… unless stated otherwise. Just my opinion.