I'm primarily left handed, but "switch hit" for a lot of things, including mice. I've never had any problem with the position of the buttons in either hand, though when using a mouse with my left hand my hand posture is at about a 45 degree angle to the mouse so that my index finger is still resting on the left button. I then rotate my hand to move my index finger onto the right button when I want to use it. When I'm using the mouse as intended in the right hand, I use two fingers. I find either approach works fine for me.

The only thing I've consciously forced myself to learn to use right handed are firearms. Left handed firearms are hard to find, and using right-handed ones with your left hand creates a lot of hardships.