It is mentioned in So You're Going To Wear The Kilt. that it is a legend or old tradition that says the ribbons indicate one's relational status (untied ribbons the heart of a young man free to admire the ladies.) and tied ribbons to indicate a married or engaged man who is bound by love and honour to his chosen mate.

But the ending conclusion is that although it's a fun story and you can subscribe to it if you like, (I do for the fun of it) he just recommends cutting the ribbon off.

And most "traditionalists" (I use the term lightly) insist it should be tied properly.