I'm left handed. But when I use a mouse, I noticed after decades that I'd always been controlling them with my right hand. Attempting to switch now feels as awkward as trying to write with my right hand.

So I guess the answer is, it's whatever you're used to. Using my right hand for mouse-related tasks was not something I decided on consciously, or due to convenience, since early models (I'm talking C=64 era) did not have the handed designs of today's mice.