Using my sandbed from old to new tank?

[The Guy]

I've done a lot of reading regarding using old sand beds from one tank to a new set up. I did reuse my sand bed when I changed from my 50 to my 90 as well as all my live rock with no problems, and plan on reusing some of the live rock with my downgrade from my 90 to my 66 g RSM and at least 40 gallons of the old water.
My concern is with the sand bed, some say use all new sand, some say reuse the existing sand or new sand with some of the old to seed it. What about new bagged live sand? I have a mixture of crushed coral and med/fine white sand in the 90 now, should I rinse it in R/O and just use it or ?
I don't want to cause a cycle and risk killing livestock, so I'm asking for some advice and opinions from my friends on Canreef