I have to admit, I undercut by 1 credit.
It's much better than undercutters who bottom out the price by several thousand credits.
Depends on the pricing. I remember selling a blaster from a Cartel pack that was going for 500k on the GTN. I initially set the price for 500k too but two days later it was returned after expiring. Checked again, people were selling it for 500k again, so I set it for 250k. Boom. Purchased. So I guess the price has to be something someone will pay credits for in the first place, then you should worry about undercutting.
The people who sell even artifact-level luxury fabrics at a loss are still annoying. I try to make sure the prices are above the default, though not ridiculously high. (10x the default is edging towards the ridiculous price levels, IMHO.) Similar things seem to happen to lvl 5 and 6 sliced tech parts.