. Just think of all of the condensation trapped behind a toilet tank fastened to the wall.
This would be my concern as well. I was taking off a countertop today in a bathroom (less than 15 yrs old), and noticed that the toilet tank had a lot of condensation on it, as I had to work around it to unfasten the counter. Yes it was an insulated tank. While it may not be a problem in some cases, it will in others, especially if this was to be a standard practice.
In a cosmetic sense I think it looks a bit funny for a north american toilet to be tight to the wall like that. I'd think best reveal is about an inch off, that is what seems to be most common in my experience.
At the end of the day i'd ask the customer, if they aren't happy another toilet could be selected. I've never had it or heard of this being an issue though.