It isn't for "peace of mind". If you have a wetroom then you absolutely, categorically, it isn't open for debat MUST tank it. You MUST.
Grout is NOT waterproof. Water DOES pass into and through grout. Depending on what tiles you plan to use they may be porous (most natural stone, porcelain albeit much less than stone, but more so than glazed ceramic).
Tank the entire floor, and tank the walls in the wet zone 1 (the shower area + 1 metre either side, and if the shower is on an outside wall then tank that whole wall).
Gypsum plaster is water sensitive. WHEN (not if) water penetrates the grout the plaster will get wet and over time start to crumble.
I thought from Jan 2012 the EU harmonised bathroom
regulations under ETAG 022? Does this not apply in the U.K.????
Tank it. Tank it now. Or I'm calling the tanking Police, and you, your plumber and your builder will all end up on Crimewatch :lol: