QUESTION: Every time I am done cleaning my katana, I put it back into my saya, sometimes slamming it in and sometimes putting it carefully. By slamming it in, I've heard that it will break the mouth of the blade or somewhere around there. My question is, will it really break something if I slam my katana back into the saya? Or is it okay to do it sometimes?
Yeah, there is a reasonable change that it could break the koiguchi (saya mouth). If you look at most JSA (Japanese sword arts) noto is performed in a very controlled manner, keeping awareness in case it needs to be used again. So they weren't really designed for slamming.
You can probably get away with it from time to time, but it isn't encouraged.
Hope this helps.