What size grout joint should I use?

It's your tile and your room, Do what you like! Do listen to the advice of your installer ,but at the end of the day you are the person that has to live with the installation. Beauty is in the eyes of the beholder and the decision on both tile and grout width should be made by the owner.

CERAMIC TILE INSTITUTE OF AMERICA, INC. says,"If recommend widths are wanted consider the following":

Decorative handcrafted tile 3/8 to 1/2 inch.

Mounted, handcrafted tile in 3 x 3 and 4 x 4 as mounted, 1/4 inch.

Glazed wall tile, as dictated by the lugs or 1/16 inch.

Ceramic mosaic tile as mounted, usually 1/16 inch.

Quarry tile and paver tile 1/4 inch.

Large sized of precision made tile, such as 24 x 24 inches, 3/16 to 1/4 inch.

Large sizes of handcrafted tile 1/2 to 3/4 inch.

Marble tile and granite tile 1/16 inch minimum and 3/16 inch maximum.