... just one more suggestion - make sure your fabric is straight of grain .. if the fold is slightly off, has kind of a kink or wrinkle when you hold it up by the cut edge, you will not get a straight strip.
If it's looking kind of iffy - iron out that center fold line, hold it up again and sort of shuffle the fabric if needed so the 2 selvedge edges line up... you can then actually fold it 2 times and still get nice straight strips - but practice on a few before you do that :)