Please don't start spreading panic. Not ALL triangle templates are out of whack. My 8" 60º triangle ruler is a perfect 60º at each angle (I just measured them) and I've never had any trouble matching seams or blocks and having everything lay flat. Take a protractor when you go to buy any angled ruler and you'll be sure.
Did you think to consider the edges may be shaved off by rotary blades? That would definitely skew the angles.
ETA: My ruler is made by Creative Grids (made in the USA, btw) and is non-slip.