You need a grout line in those just like any other tile. It does not change the process much except you must either use an slc to level the floor after installing the heating or be very diligent about getting the floor flat as you set the tile. Any good modified thinset will work just fine.

The product you’re speaking of is an acrylic-based caulk used to finish up the installation. Per standards any change of plane, that is where horizontal and vertical surfaces meet as you’ve mentioned, as well as corners where two walls meet and anywhere tile meets a different material such as the acrylic tub, must be filled with a flexible sealant. These include the caulk which works fine, or color-matched (or clear) 100% silicone, which works better because it remains flexible longer. It should not be used in the entire installation.

Any caulk or silicone is not made to be a permanent product, all of them will dry out over time and need to be replaced periodically (I’m talking about years here). Once it does begin to dry out it shrinks a little bit at a time. This causes the seal between the caulk and tile to break loose and open slowly. Once that happens it should be replaced. It’s completely normal but most people simply fail to replace it when needed. I’m sure you’ve seen the clear beads of silicone in tubs with all the nasty colors behind it? That’s why, the silicone shrunk, seal was compromised and it wasn’t replaced – nice little breeding ground for all sorts of nasty stuff.

Caulk will normally last about 3-6 years, silicone goes about 5-8 years. Silicone is more flexible and lasts longer. Now that I’ve totally grossed you out on a Sunday morning I’ll go away and leave you alone.