No it doesn't leave holes to skip a st when picking up. For example, if your gauge is 4 sts and 6 rows per inch in stockinette, that's the 2 to 3 ratio. If you pick up in every stitch, you'll get way too many sts along the edge (which is the end st of the row) and it'll be ruffley. If you pick up every other st, that's too few and the edge will pucker. Garter st ratio is 2 to 1, so you can pick up every other st there. Slipping the edge sts is more likely to leave a hole than not I've noticed; picking up in that slip st can pull the next st and leave a gap between the edge and the body.
About the neckline... Most people will say to do the inc one st in from the edge so you have a stitch to pick up in. I don't, but you can still pick up in it anyway. For the back neck, you pick up every stitch, the same for the front if the sts are sort of level rather than like a V neck. But along the sides you would probably have to do the 2 for 3 pickup as it's slanted rather than straight up and down. Practice it a little - CO 10 sts and work a few rows with a dec on one side for a couple inches. Then pick up all the way around your piece so you see how it works on a slanted and straight edge - picking up in the CO edge you'd pick up every st.