I yarn under on most of my amigurumi. It lets me get a tighter stitch without using as small of a hook. Plus, it is a lot safer for little ones because the stitches are tighter.
I prefer the "x"s look. Its looks proffessional and more uniform somehow, while "v"s lost the form.
The vs are like "puff puff" ヽ(>◇<。)シ... Puffed out, while "x"s looks flat and neat.
The x stitches are so hard on my hands! I learnt doing V stitch so to change to x is really hard and I end up getting frustrated and going back to V stitch halfway through