Flooring Tile - batten board,or not?

i am going to start/try putting up 6x6 ceramic tiles over 1/2" hardi backer, around a regular tub-5'x5'-back wall & 3'x5' on the ends.
I see some talk of using a method of supporting the rows of tile with a horizontal support,then install the boottom row later.
my question?
is this necessary/recomended?
or just put a 1/8 shim/spacer along the top-tub rim?

If the tub is dead flat level on all three sides then you can get away with the shims but I haven't seen one yet that is. The board will give you a sturdy support structure to carry the weight of all the tile above it while the thinset sets up overnight. I would set the boards so my first full tile off the tub deck is resting on the board. The bottom row of tile will all be cuts, usually try to get the layout so you are only taking off an inch ot two. Check the vertical layout to see what kind of cut is left at the top of the wall, you want something manageable up there too. Use spacers to keep the rows of tile from sliding down on top of the ones below.
There are some new thinset out on the market that have superior gripping characteristics but they can be a little tough to find. Laticrete 255, Tec 3N1, Custom Mega-Light are a few that I am familiar with. Using these thinsets can negate the need for the boards but there is a learning curve with mixing them to the right consistency so the tiles dont sag on the wall.

thanks for info
i will do some more measuring/calculating.& put up some plywood 3"strips around the tub?