The only time I've had to be careful with the amount of water changed was with my cherry shrimp- and a lot of that was because, at the time, I was not adjusting the GH until after - I started adjusting GH prior to putting the fresh water in and all was good.
All my other fish either love soft water or tolerate it (which is what i have), so I do heavy, regular water changes. 100% would be fine, imho, but too much of a PITA to do LOL....and not really necessary for the situations I have. I would think a betta in a jar would benefit greatly from 100% change periodically.
I am surprised at the length of time in between changes, though, I have no experience with bettas like that.