I am putting ceramic tile on my bathroom floor and bought 1/4 inch backerboard. I didn't put thinset underneath the backerboard between the tongue and groove sub floor. I did use the proper screws to anchor down the board but I am worried about the problem of not having thinset underneath??

skiberger

The toungue and groove, this plywood or old t&g 1x boards? If over plywood you'll be ok. Just make sure its secured well. If over 1x boards it is recommended that thinset be used. The thinset helps bond but also "level" the 1/4" backerboard over uneven types of subfloors.
I thinset 1/4" all the time. If I used 1/2" it depends what type of floor I'm going over if I thinset it or not because its thicker and wont follow the "contours" of the subflloor as much as 1/4" will.
So basically if you subfloor is level and smooth, dont worry about it. Start tiling.