Good idea to tank walls over a shower bath?

[Qwerty]

No. You can mark a fixed line then cut tiles to this for first course instead of battening off.

 

[Stef]

As stated always fit your bath 1st then tile onto the lip of your bath.
Silicon the bath to the wall then Silicon the tiles that way you are creating a double seal that will never leak water.

 

[jollyroger]

Just mark a straight line on the wall and fix your 1st row of tiles. You can use small packers or wedges underneath to raise them to the line. Try not to leave too much of a gap though, 3mm max or your final Silicon seal won't look too great. Good luck.