Sometimes I'll start things off using my left hand, however, when things get fast and furious, it's all right handed to completion.
If I'm jacking off while I'm in the shower or bath tub, I'll typically use my right hand, especially if I'm using the handheld shower head, it's in my left hand. On the other hand (pun intended), when I'm at the computer, I'll masturbate with my left hand, while I manipulate the mouse or keyboard with my right hand. Left hand until just before I cum, then I'll switch to my right hand for completion.
Ambidexturous in everything. And with self pleasuring. Makes no difference which hand is used. Get the same fantastic result either way.