If you can't get to it fast, it should stay home.
It'd be a great shame for an errant haggis to suddenly jump from the trencher and escape because your dirk was stuck in it's scabbard!

Most folks I know wear their dirks on their right hip, far enough aft so as not to interfere with your right arm's swing as you march. Lots of fellows I know wear one with their formal dress; it's handy to have a frog that you can detach the scabbard from, as it's quite awkward to sit to dinner wearing a dirk. Burns Night and St Andrew's Dinner there's a line of dirks down the center of the table.