I always back butter my tiles. Many people do it wrong, however. Heres how I would do it:
Put a small amount of thinset on the back of the floor tile and scrape it into the recesses of the tile with the flat side of your trowel.
All you're doing is filling these recesses, you don't "notch "it.
Then when you lay the tile there's no buildup of thinset to deal with.
This allows for the thinset on the floor to bond nicely with the tile.
If you are skeptical, do it with and without this method and then pull up the tile.
You'll see the buttered tile has greater adhesion.
It's also super important to mix the thinset properly and to only trowel on as much as you can cover in about 10 minutes.
When the thinset starts to "skin over" you will need to scrape it of and apply fresher stuff.
PS. Don't walk on the tiles like the guy in BJ's video.