When I have a new gi that I want to break in and soften, I put in about 1 - 2 cups of white vinegar to the wash cycle. I repeat the agitation cycle several times and let it soak for a few hours.
For regular maintenance, the white vinegar is typically added to the rinse cycle. About 1/2 cup.
I do this whenever I have the white vinegar on hand and remember. You can certainly do it with every wash with no fear of harming your clothes (much gentler than bleach).
It's a great inexpensive and natural alternative to fabric softener, which can leave residue on the gi and interfere with its moisture absorbing capabilities.
Thanks a lot. I had heard of the practice before but nothing specific. Appreciate you taking the time to break it down for us.