I would suggest heavily spray starching the top before layering, and also heavily starching the backing. This helps stabilize the fabric and prevent stretching/distortion/puckers when quilting.
I would think diagonal lines would be more difficult. For one thing, you would be mostly sewing on the bias of the fabrics which makes stretching/distortion of the top and backing more likely. For another, you would be crossing a lot of seams which makes stitching problems somewhat more likely.
What I really like (and it has the advantage of also being easy!) is wavy lines across the quilt. Here is an example:
http://imperamagna.blogspot.com/2012...-finished.html
This is especially nice on modern quilts; might not be as attractive on vintage style fabrics. To achieve this look, it really helps to use a walking foot. All you need to do is guide the quilt sandwich from side-to-side to create the waves (it is a straight stitch), and the waves definitely do not need to be equal distances apart from each other.