Grout is the glue that locks the whole installation together into a whole mass and makes it strong. Without grout, the whole floor is much weaker.

Tile is also not perfectly straight and if you try to butt it together, your pattern gets off and it will show up as wrong. Even rectified porcelain, which has the straightest edge of all tile cannot be butted together. But if you do want minimal grout lines, that's what you need to choose. And the really important part is to make sure that your joists are sized for tile. They need to be robust enough and deep enough to minimize deflection. Or if you are slab foundation, you need to make sure it's very very flat. It's like a paint job. All of the work is in the prep for the pretty covering.

Thanks for the response.....but I have a question, so when building I need to know if I am putting in tile or not so the joists are sized correctly? I have never heard of this? I guess you learn something new every day!

And the larger the tile, or if you use natural stone, the stiffer the floor needs to be. Or the tile will crack. So yes, you need to discuss the finished floor material with your builder so that everything can be planned out. And if you are going to choose materials with different heights, it can all be accounted for if your architect knows then while designing the home and you won't deal with those horrible transition pieces at thresholds that bridge the height difference between the wood and the tile.

I would direct you to a site that has a calculator that determines deflection rate,i.e. the bounce a floor has. But alas the moderators here have had problems with security from that site, good to know.

Can't have any bounce on tiled floor, the tiles and grout will crack.

This is simple deflection rate test. Put a half full glass of water a few feet from a wall in the room, go to the centre of the room and jump up and down, really jump, if the water in the glass moves you can not install tile in that room without doing some beefing up of the substrate.

I' not pulling your leg.

You can Google JohnBridge Forums and they provide a "Deflecto" calculator in the blue bar at the top of the page that should help.

As to grout width, measure several tiles to find the widest variation between smallest and largest. Three times that variation is the minimum grout width allowable.