Some people use a triangle trimmer template/ruler such as this one:
http://www.frommarti.com/trimmer.shtml .
When you cut the triangle points off before you sew, you then have a perfect flat end to line up with the side of the opposing triangle. No guesswork, and for my money, easier than drawing a line on every triangle. After a while, you will get used to eyeballing where to line them up. I know I did.