It’s technically possible to do a grout-less tile installation, but tiles should NOT be installed without grout joints.
The primary reason to avoid grout-less tile installations is Expansion & Contraction.
Normal expansion & contraction that occurs within assembled buildings causes movement. This movement is accommodated by grout joints that compress and expand during movement. Tile does not have the flexibility to move and if grout joints are not present, tiles are prone to “tent” or lift off the substrate due to movement associated with expansion & contraction.
The ANSI specification regarding the minimum grout joint size (A108.02-4.3.8):
“To accommodate the range in facial dimensions of the tile supplied for a specific project, the actual grout joint size may, of necessity, vary from the grout joint size specified. The actual grout joint size shall be at least three times the actual variation of the facial dimensions of the tile supplied. Example: for tile having a total variation of 1/16″ in facial dimensions, a minimum of 3/16″ grout joint shall be used. Nominal centerline of all joints shall be straight with due allowances for hand-molded or rustic tiles. In no circumstance shall the grout joint be less than 1/16″.
If one of your clients insists on skipping the grout, keep this in mind: the tile industry does not support grout-less tile installations. If you end up in court due to a failure caused by the lack of grout joints, you will not find any support from tile industry experts that may be hired as expert witnesses. Talk the client out of grout-less installations or decline the project.