Here is what my Viking Dealer said about straight stitching on old versus new machines.
A straight stitch sewing machine will ALWAYS have a nice straight stitch because that is all it does. Any machine that has the option of a zig-zag stitch will never have a perfectly straight stitch. Even when you set it for straight stitch it will stitch in an ever so slight zig zag stitch. It's the way they are made, and there is no way around it. If you want a perfectly straight stitch on you sewing, then use an older machine with only straight stitch capabilities.