When I'm looking at porn on the web I usually stroke with my left hand because I need the right hand to use the mouse, but I usually finish myself off with the right hand.
Ambidextrous actually means having the ability to used two right hands. If you go to the origin of the word you'll know what I mean. People have just picked it up as being able to use both hands equally.
Same here. I often use the left hand for masturbatory foreplay, keeping myself going, until I get to the point when I feel I need for something more. At that point I switch hands in order to finish off. It's very rare when I've been able to climax by using my left hand alone.
When I was 16 I cut my right index finger off, and had it reattached. My hand was useless for a couple months. It was then that I learned the importance of being able to jack off with both hands.