More:
I usually like to wash my fabrics before cutting them.
Single knits will curl on the edges when dried in a dryer. Maybe sewing the ends together will help minimize that? So - I don't really know what to recommend - I wash - and fight with the curling edges. Not totally sure it's worth the battle.
Double knits are a dream to work with.
Stretch fabrics like heavy spandex types of fabric are challenging for me - skipped stitches, etc.
Those pretty fabrics with the mini-sequins printed on them? Cute - but I have had trouble with skipped stitches using them.
For older people - I usually put two rows of stitching on the crotch seam - 1/8 inch apart.