I have heard to use a high quality silicone between each piece of densshield around all joints and corners and also a dab over each penetration including screw heads, around shower niches, around the mixing valve, and around the shower head outlet. A true silicone never really hardens and this would form an elastic property when I tape and "mud" the joints, also per densshield directions.
Is this a concern? Obviously the silicon should be used with modesty just enough to seal the joints but I will be applying enough that I am comfortable it's watertight.
Best Answer
Your silicone seals should never be wider than about 1/2", and you should avoid smearing caulk onto the face of the backer board. Your mortar and tile will span those small gaps without an issue. Keep in mind that the boards should all be securely fastened to the framing such that there's no movement with respect to each other.