I would guess that most people don't bother with tags. I usually do if it's my turn to be the OP of a serial thread, and I've used them a few other times. I try to make them serious, or at least if I use a joke, it still has something to do with the topic of the thread.
Tags can sometimes make all the difference in finding threads. Thank goodness most people at TrekBBS used them when they were the OP of the avatar contests in one of the subforums. Years later I asked for a master list of themes for these contests to help keep people from duplicating them, so one of the mods there helped me find as many old threads as possible to compile a master list (there wasn't one before). We had to dig back years, but finally found them. I compiled the list and it is still being used and updated.
EDIT: One of the "similar" threads up for me now is "Identifying Lost Mods". Well... I can think of quite a few who vanished and we haven't seen them in years...
