Bathroom remodeling question

I am remodeling a bathroom in our house and the room is now gutted. I am planning to tile the floor so I know I will be putting backerboard over the subfloor. My question is do I need to put backerboard under where the new tub will go, or can it rest directly on the subfloor?
The bathroom is small, about 7x7, so putting in the backerboard isn't really a problem, just wondering if it is necessary.
Thanks for your help.
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