You likely don't want anything on the threads with a slip joint. Now, on a sink tailpiece, that can be a different story - the gaskets used and how they seal are different.
Since you have access, my take is to use the cemented connection. When setting the tub, make sure it is level and properly supported. If the subfloor isn't perfectly flat and level, and the tub rocks or warps, it can create long-term problems. Whether it requires shims or bedding in mortar would depend on the tub and the floor. If you're lucky, it has a full support and your floor is flat and level.