My recommendation depends on how many tiles are involved. If you have few enough that a dab of something on the back is not too tedious. That would be fine. However I would recommend construction adhesive over over silicone. It will have better holding power. I often use PL polyurethane construction adhesive.
If there are going to be too many tile to dab the back of each, I would mask the portion of the mirror that will remain visible and spread tile mastic with a 1/4" plastic V notch applicator.
I hope this helps.
The Following User Says Thank You to Portlandtrades For This Useful Post: