I have the Fons and Porter 1/2 inch ruler that you draw on both side of and then stitch on the lines. Well even if you stitch exactly on the lines you can still come up crooked. It seems to be with the fabric itself since you are sewing on the bias. This is for half sq triangles.
Strips I no longer have a problem with since I am very gentle with them when sewing togather and pressing. It also helps if you have lines on the pressing mat to line up the strips on.