Ok, I'm sure that MANY folks already know this, but....

when you change strings (when strings are fully removed from tuning post) make sure that the hex nut is tight as well as the little screw on the other side.

I first learned of this when my acoustic bass had a lack of brightness on the A-string. You can't really tell that tuners are loose with string tension applied, but once I removed strings, sure enough - they were ALL loose!

This may (or may not) cause noticeable tone loss (maybe eventually). But it's certainly worth it to check and tighten.

I was recently restringing my acoustic 6-string and again, all 6 tuners were loose!

This is just something that's easy not to notice, so you are hereby notified

CREDIT: I learned this on Frets.com