McP77

5mm is bigger than average for wall tiles but I wouldn't say it was wrong. The most important thing is that the client was happy with the result. As for the edges of the tile showing I'm not convinced the size of spacer had anything to do with that.

Spacing is a personal choice. I personally prefer using a 2mm joint on wall tiles, others go for 3mm. I think the thing to remember is to offer the client the choice. As long as it's technically feasible then it doesn't really matter what client chooses.