A setting bed of mortar is required on top of the liner. There are lots of little details that need to be done right if you want this to last and work right. Suggest you check out www.johnbridge.com for detailed help with tiling your shower.
If done right, the top edge of the mudbed pan should be even all the way around the shower. Unless it is square and the drain perfectly centered, the slope will be different on the short verses long edges. The long slope needs the 1/4" per foot, which means that the short slope will be steeper to keep it even all the way around.
Thinset will NOT stick to the liner...you need the setting bed.