Yeah it's not the done thing. There are the odd slate mosaic strip things (can't for the life of me remember what we would call them but they have a name) that have such an uneven surface it's almost impossible to grout so they're often butted up. Walls only mind and usually just one part of a wall or a feature area. Sort of okay in that case but still likely where something will fail 20 years down the road.
Generally it's 2mm minimum for walls and 3mm minimum for floors. And that's with small (compared to now) 12×12" floor tiles etc. With even bigger tiles you may need bigger grout joints because of the expansion.
As touched on above thin porcelain expands about as much as thin glass. They're nearly similar in density and whatnot.