A good straight stitch is not really a straight line. On a lock stitch machine there are two threads wrapping around each other. The amount of distortion produced from this wrap depends on several things: the tread, the fabric, the needle, and the tension.
Jenny over at sew classic has described this beautifully. Look at this on her site:
http://blog.sew-classic.com/2008/10/...-about-it.aspx
or
http://tinyurl.com/6286of
Cathy