General tiling options

[J Sid]

I say it's not a must but very advisable to both protect wires and help distribute the heat

 

[Dumbo]

If you get air pockets around the wires the heat won't dissipate properly at that point and the wire can burn out . Latexing greatly reduces the chances of this plus alsp there is less chance of damaging the cables during tile instalation

 

[hmtiling]

The elements need to be fully encapsulated as above. It can be done with adhesive when tiling but this will take longer and will there's danger of damage to a cable.
Also, if ever a tile needs changing there's much more chance of doing so without damage to the wires if latexed. Madness not to latex imo.
My preferred method is same as Julian apart from mat instead of loose wire