You really dont' want to mess around when it comes to water. Ask anyone that's had a leaky bathroom before.
The BEST thing to do is to have the tile installed on a smooth, flat surface. Cement backerboard is fine. Also, if there are any outside walls, be sure you have insulation and a vapor barrier installed first.
My best to ya and hope this helps.
Jay J -Moderator
PS: For some reason, you didn't tell 'us' why you want to leave the wall 'uneven' ...