I am renovating my bathroom and am getting close to tiling the shower. I have the hardibacker in place and mortared the seams etc. The guy at Home Depot made it seem like I needed to apply a waterproofing before laying the tile (RedGard 1 Gal. Waterproofing and Crack Prevention Membrane). This stuff isn’t exactly cheap but I don’t want to end up with a leak somewhere. Is this stuff needed or was the rep just trying to make a sale?