In regard to gathering - in order to lock the stitches so they don't come out - you need to backstitch or lock the stitching. (Another way is to tie the thread tail in a knot.)
1. Serge for about 1" and then bring the thread tail around in front of the needles and knife and serge on top - over it.
2. When you get to the end - serge off about 1", then flip the fabric upside, bring around to the front and stitch on top of the previous stitches about 1" and then serge off.
This will lock the stitches at the beginning and the end.
Sometimes I don't do this because I want to ease the gathering or modify the amount of gathering.
It is suggested that if you are putting in a cap sleeve or any sleeve you can gather between the dots to ease the sleeve into the armhole.
SPECIAL NOTE - IF YOU ARE ATTACHING A GATHERED SECTION TO A FLAT PIECE OF FABRIC - THE GATHERED FABRIC MUST BE ON THE BOTTOM! Otherwise the gathers will flatten out. I know because I have made that mistake once or twice. It was just as painful as stubbing you toe.