To answer Mephis88's earlier question about where to obtain good-quality Tartan items, I'd recommend the
Scottish Lion of North Conway, N.H. and North Carolina.
I've followed the example of my father and grandfather and ordered from the Lion for years. They carry a full line of Locharron products and, for most clans, tartan products in the various ancient, weathered, dress and modern variations. In particular interest to members here, they carry caps, scarves, ties, bows and cummerbunds among countless other accessories.
They're quite reasonable and, from my experience, easy to deal with by phone and over the Web.
In terms of etiquette, I'd concur with NavalGent's assessment. While not all of my groomsmen are members of our clan, they all wore ancient Forbes ties in dupion silk from Locharron.