"...Basically I have been sewing most everything at the same length henry Shires uses on the tarptents. Is this bad form?"
Generally, in custom made products, yes. Bad form means you will be coppying the manufacturor's production length. For sewing large volumes of fabrics together with a rolled seam, most manufacturors use one length, it is easier. Doing the same with rolling a mesh into a silnylon seam is OK, but alters the strength of the seam. It is not the same, nor should be treated the same while sewing. Adding a zipper? Or sewing a dor over mesh and canopy? One layer, two layers, rolled/felled seams, will require different stitch lengths. Attaching webbing loops to fabric requires a different stitch length, often a different needle, thread and tension.
Stitch length, needle size, thread size and tension are all related for a perticular bit you are working on. Some companies prefer cotton wrapped polyester threads. Some prefer extra strong nylon threads. Some use 100% polyester threads. This will effect the size and shape of the needle. The size of the needle will effect the strength of the substrate (along with stitch length.) Different materials do require different stitch lengths. Differnt construction methodes require different stitch lengths. A glued seam will require not only enough to hold everything firmly together, but will also reinforce the glue making thread, material and glue work together...a balance between several components, not only stitch length.
Sewing burlap is much different than sewing silk. Stiching leather will not work well with the same seaming used for sewing silk. Sewing cuben can be a disaster when compared with glueing the same seam. PU fabrics loose waterproofness and strength with the same seam as used for silnylon. Just a few rather drastic examples of how thread size, stitch length, glueing, tension, needle holes can influance seams. This is besides the obvious number of perforations that the needly puts in the fabric.
I don't believe there is a chart of these things, except for generally. There are simply too many variables. This is part of what makes fine custom sewing so different from factory made stuff. More art than science. Watch a good taylor at work. He will often do almost incomprehensible steps that take time. Neat and consistent make strong seams, generally. Long stitch lengths have their place. Short stich lengths have theirs. When do you use them? Well, it depends...