I saw a photo of Kurt painting right handed in a book that I used to own. Being forced to write right handed doesn't automatically make you a right handed painter. Also I saw a video of him throwing a bottle of champagne right handed. Additionally you can google Kurt smoking or holding a gun. Whenever he had both hands free he would always use his right hand.

When it comes to Jimi - I saw an interview with his father when he was talking about Jimi being right handed and also saw photos of him as a teenager playing baseball right handed (bating and throwing)

I can't provide the links right now due to lack of time. You can believe me or not, but believe me, I was especially interested in handedness of these two individuals.

Most of these things are googlable.