I'm learning the short answer is "YES"!
If you have a lot of time and $$$ invested in your display tank...and would not want to have to break it down to "deal with something"...then yes QT everything!
I'm thinking I may need 2 QTs!!!
I had some things I got from other folks in QT when I moved a RBTA split (that I damaged his foot trying to move him) and put it into the QT.
Now my nem is healed, and no longer a danger of turning to slime in my DT...and I've dipped most things with CoralRX...but not big hawian feather dusters, nems, snails, etc...and now I've got flatworms in my QT!!!
Good news is not in my DT, but I'm not sure I trust moving the RBTA nem back to the DT. Feels like I need another fresh QT tank to house the nem alone for a while...
Anywhose, I'm sure snails, chaeto, nems, even plain ol frag plugs can introduce problems.
QT is pain, but the best way to reduce headaches~