Solid Hardwood, Engineered and Laminate Flooring - Thinset or mastic necessary between subfloor and backerboard?

Everyone has always been very helpful on this forum, so I'm hoping someone can answer me this question. I am remodeling complete bathroom and am ready to do the floor. Will be laying 8 inch tile on cement backerboard above subfloor. So, question is: I already started to screw down my backerboard on the subfloor whan I read in a book to lay a layer of thinset or Type 1 mastic between the backerboard and the subfloor before screwing it down.

Do I need this layer of thinset between the backerboard and subfloor? It would be no big deal to unscrew the backerboard I've already done and then lay it down again. But, is this necessary? What real purpose does the thinset provide if I'm screwing the backerboard down anyways?

Bathroom is small, about 20 sq feet of area I'm tiling on the floor. 2 sheets of 3x5 backerboard were plenty to do job. Subfloor is in excellent shape, and will be laying down 8 inch tiles.

Also, if it is necessary, what is best? Mastic, construction adhesive, thinset?
I hate to buy a huge bag of thinset for such a small area and mix it.

Thanks in advance for any help!

Mike

Go to www.doityourself.com/ceramic for helpful tips. A flexible thinset mortar is recommended as well as proper subfloor prep for proper installation.

We used the mastic, but I have since been told by several people that it is not necessary. One of which was the guy at the tile shop.