Well that depends on the tile and on the size grout joint you prefer
A lot of 6 x 6 tile has 2 knubs on each side of the tile which are designed to act as built in spacers. If they have these you can just butt them together as you install them.
If your tile does not have these you may need to use spacers. Generally a tile that size will be installed using a grout joint about 1/16" - 1/8" wide. If you install them this way you will use a non-sanded grout to fill the joints.
Most wall installations will use joints 1/4" wide or less. If you decide to use a joint larger than 1/8" in width a sanded grout will be used to fill the grout joints.
If you have any additional doubts, go back to where you purchased the tile to see if they have any installed examples to help you make your decision.