From experience tiling directly on to the sheetrock can cause some issues. We have earthquakes in Alaska and i have seen a lot of decorative tile pop off of the wall because you are basically gluing tile to a piece of paper that is covering the gypsum. Walls move and settle and that amount of weigh can cause the paper to rip.

Using the backer board the tile would get better adhesion since that is the purpose of the board and the weight distributed is more even. Using the 1/4" should be more then enough since it is not going to be used in a shower and you are not using it as a structural part of the application.

That's really good info. Thank you! We're in Georgia. I know we get an occasional earthquake, but it's once every several years. Sounds like the backer board is a safer bet. Would you screw it on the drywall, or completely remove the drywall and install it?

I've seen plenty of tile jobs directly over drywall and holding up just fine. If there's no moisture around, like next to shower or sink, I'd go over drywall. An option is to replace existing drywall with moisture resistant (or whatever they're calling it these days). Backer board is overkill out of a shower or tub enclosure in my opinion.

If you want the tiles to hold up for a long time I would suggest using the backboard. Its not necessary but I always believe if you are going to make an upgrade that you should get something that will last longer than you may even intend for it to.