yes it matters...
no it doesn't!
if you've been consistantly adding 2 stitches (1 for either edge) then continue to do so.. (most sewn together afghan blocks will 'lose' a stitch in the seam.
Does it matter? it matters sometimes..
Just as how you 'handle' the selvage stitch matters sometimes.
MOST US patterns have a 'built in selvage stitch (just as US sewing patterns also include a seam allowance)
MANY (used to be most, but things are changing) European knitting patterns often don't include a selvage stitch.
you have to read notes (about the pattern)
Your afghan could end up a few inches bigger than expected (usually not a problem!) but since an afghan doesn't have to 'fit' its not really an issue.