As a rule we go by this when selling, something we got sent a good few years ago.
Planometric Tolerences
With the increasing popularity of large format wall and floor tile we wish to remind you that the tolerances allow for a 0.5% warpage in tile planarity. therefore the european norms allow on a 60x60 tile 3.5mm tolerance between the centre of the tile and its corners.
This causes no problem whatsoever when the tiles are fixed in the normal draughtboard patern, however, it can create problems if the tile is fixed brick bond or half bond particularly in the case of rectified tiles when the joint width is reduced to 1.5mm. It is advisable in this instance to ask your client to check for acceptability BEFORE fixing or alternatively to consider bonding 1/3 - 2/3 which will have the effect of halving discrepancies in planarity.