It depends on the application. When installing DekTek Tile on an elevated deck exposed to the elements, you DO NOT want to grout the tile. The tile joints are kept open so the water can drain through the deck.
If you are using DekTek Tile in conjunction with a “waterproof decking system,” then you may want to grout the joints, depending on the type of waterproofing system you use. If you do grout the tile joints, your deck should also be pitched down at the edge so water will drain off.