Due to my height (and extra-long torso) when I wear my custom length tweed Argyll and waistcoat with the kilt, there is enough visual separation of a tartan tie from the kilt that it is usually not as problematic as others here have stated concern for, although I still try to shy away from the practice and instead wear a small patterned tie with similar color scheme to the tartan in the kilt. I think a lot of it also depends on the "visibility" factor in the tartan in the kilt---the more visible the tartan the harder it is to hide the tartan tie--whereas some more subtle tartans can stay in the background as a tie even when the kilt is a prominent but also subtle presence in the overall outfit.

This is me a couple years ago in Ancient Douglas kilt and tie with Lovat Green Jacket and waistcoat, IMHO not that offensive due to the waistcoat adding to the separation between the tie and kilt and minimizing the amount of tie visible at all:



One other problem is that a lot of tartans used for kilts are woven at standard sett size but the ties in a lighter weight material at a smaller sett size and by a different mill (most commonly Ingles-Buchan), using slightly different color palettes for the same tartan than the kilting material. This makes color matching difficult, and as others have said slight color variations between parts of your attire can really make an otherwise nice outfit look a bit atrocious. Another good reason not to wear a tartan tie with the same tartan kilt.

Okay, now point and laugh at the silly american in the picture. Just remember to notice no white hose, no flat cap, and no bonnet in the house. At least I got some of it right.

jeff