It depends on the fabric you're using and the look you want. While 5/32 welt cord is the standard for older cars, the newer cars that use welts generally are smaller than that. On automotive seats, I generally use 4/32" plastic welt cord when I'm sewing vinyl, leather, Ultraleather or thicker cloth fabric because then it won't look so large. Basically, this is a personal preference on my part. I don't like the welt being the focal point of a seat cover, and if I can, I choose not to use welts at all. They are the first thing that will wear out on a seat cover.
You also need to use the correct size welt foot on your sewing machine. The rule of thumb is to use a welt foot 3/32 larger than the welt cord, so for most sewing of 5/32 welt cord, you should use a 1/4" welt foot. Naturally, thicker fabrics can make a difference, so that same foot is good for 4/32 welt cord and thicker fabrics.
No one lives forever, the trick is creating something that will.