Need? well, you can do whatever you want but failing to provide for it to drain will result in a pool of water that is unwelcome for a variety of reasons.
so, yes, the bench should be inclined enough to prevent standing water on the bench. If the surface is really flat, it doesn't take much but when installing tile, you often end up with an not quite perfectly flat surface so you can end up with a sunken spot. Just make sure when all is said and done, there is no pooling.