One thing I do to help keep them straight, I sew them into groups 6"-10" and make sure they are even sided, and then sew those sections together if I need something longer (like a border). I normally rough cut my strips just a bit longer than I need so I don't need to be as worried about getting them out of line. I think some use paper to keep them going straight so they don't accidentally end up with a a strip that curves.
I normally sew in pairs, and then sew the pairs to make groups of four, then sew them to make groups of 8,