There is no historical symbolism in tartan, or any other cloth, cut on the bias. Heathbar is absolutely right about the early practice of cutting trews and hose on the bias.

The making of jackets and waistcoats on the bias was a feature of the later Regency period and was in essence a fashion statement. The earliest bias cut tartan jacket I'm aware of is that worn by Col Alasdair Macdonnell of Glengarry in Raeburn's 1812 portrait. Glengarry was a bit of a dandy and perhaps started the fashion. Any style of tartan coat generally disappeared during William IV's reign in favour of plain coats although the odd one was still being worn later, probably older ones rather than contemporary with the date of the picture/portrait in which they appear.