I am fussy about tails, LOL, as I have used several model stitchers who do beautiful stitching, but have terrible tails! I always tell my stitchers, run AWAY from the edges, and clip as close as you can to the stitching, and never carry threads across unstitches spaces as they too often show on the face of the fabric...
As for how many to run under... minimum 3 as has been said, but more never hurts, LOL.
The only exception is if you are working on something that will be washed / used a lot (ie sweatshirts, table toppers or runners, tree skirts etc.). In this instance, run under 3 stitches THEN CHANGE DIRECTIONS, and run under 3 stitches in another direction. This helps prevent tails from working loose as the piece is handled or washed...