6 Hours Ago
Jonny I asked the same question of Mapei maybe 3 or 4 years ago now and was told the same thing. Can't remember the exact reason he gave me but I remember thinking logically that it shouldn't adversely effect the grout performance or result in patchy grout, so was debating doing it anyway.
However seeing as Mapei said not to, I didn't fancy running the risk of having to scrape out 30sqm of grout just to prove to myself that it could be done.
be interested to see how you get on... If you do infact end up using Mapei.