Sometimes a pattern will say 'every second knit row' or 'every second purl row,' which would essentially be every 4th row. Every other row is just that.
If a pattern says every second row, then it is
every other row, and they should have had more coffee.
Some patterns include selvedge stitches, and some don't. You can usually tell if the pattern ends and starts with a knit regardless of what's in between.
Other patterns don't call for one. I guess you can add one if you like, but I don't think it's essential.
I like to put them on scarves because it does leave a nicer edge, but I don't add one to sweaters that are going to be seamed unless the pattern calls for one.