Harry-- you seem to know more than most about tile underlayers-- so here it goes--
when tiling a shower and only the ceiling is to be tiled.. meaning, it is a shower stall so it is all composit material.. do you still think that waterproof membrane still be added..
also i've been told to use fiber tape over the seams of the backer board,after i have filled the gaps with thinset..
hmmm what is this tape? what if it's not there also do you apply the tape after the thinset cures?

when i first started using backerboard i would fill the gaps with caulking,and wipe it flush..

""thanks
novice----jeff

Actually Jeff there are a few guys here that use various products which I've yet to use but in regard to cement board units (cbu) they are very much the same in their installation process, although you should still refer to the instructions for each product.
Fiberglass mesh tape can be found in the paint dept of most hardware stores and when used for seams in backerboard you can simply use the same thinset mortar used to set the board.
It doesn't matter if the mortar behind the board has cured or not, but usually it's all done during the same time as the installation of the boards.

Caulking the seams would be the same thing as a "control joint" which means that any stress under the boards from structural movement etc would be released at these joints and possibly effecting the tiles over top these joints. This may never happen and is only a possibility but taping the joints with thinset mortar and mesh tape seems to allow multiple panels to act as one although there are still limitations to how much stress can be endured. Depending on the area being covered and the specifications of the product manufacturer you may be required to include control joints at various locations.