My pattern calls for a one stitch selvage edge. I am using a chart and the odd rows are not listed, as they are worked as they appear. Does that mean on the wrong side row I purl that selvage stitch or is it always a knit?
Selvage stitch edge
Usually, I purl the selvedge st on the wrong side and knit on the right side because it makes an easy edge to seam.
I use the simple chain selvedge stitch – always slip the first stitch, always purl the last stitch. They are not a part of the pattern stitches, just kind of a ‘frame’ that goes around. This selvedge creates bigger loops, actually every chain loop “covers” two normal pattern stitches on a side.
If you use mattress stitch to join sides, it’s probably better to go inside the first pattern stitch because those ‘horizontal bars’ are somewhat far apart… I use a different method of stitching though. Lately, it’s single crochet through both of those big loops aligned together. But it used to be sewing on the wrong side with the backstitch – going into the second loop and coming out of the first, then going into the third, coming out of the second as so on.
I wish I could provide video links cause the picture links that I found don’t explain much 
ETA: Also, if making a sweater I find it’s much better to pick up sleeve stitches from (or sometimes through) selvedge stitches. Picking up from every pattern stitches creates very puffy tops of my sleeves.