I'm with Alan P.
Spacing allows for a little bit of movement, and also gives the tile a little bit of lee way when fixing the tiles, the substrate has to be perfect to butt joint.
You'd just be making life hard for yourself.
I was taught 2mm for walls and 3mm for floors as British Standards. Just been reading through them as I have a bathroom coming up where the customer wants 1mm joints (as per porcelanosa display).
Could only find this in BS-5385:
Tiles should never be fixed with butt-joints, as an adequate width of
joint is necessary for the relief of any local stress. Joints of
approximately 1 mm to 2 mm should be left around every tile
by inserting spacing pegs of suitable thickness between the tiles as
fixing proceeds. If for design reasons wider joints are required, the
same technique should be adopted.
Getting the substrate spotty dog wont be a problem finding a tile without a noticeble size difference might be, unlike timber I cant buy it regularized. I could allways buy straight edges and run them all through a cutter with fixed straight edge first.