It is best to thaw them before using. Seitan's versatility lies in the myriad forms it assumes during the cooking process. I find simmering to be the most effective and efficient preparation method. But it can be oven-braised, baked, cooked in a pressure cooker, or deep fried. Each version yields a different texture.
Oven braising produces a texture similar to the chewy texture derived from simmering.
Baking produces a light texture that works well when grinding or grating seitan. Fried gluten turns soft and slippery when cooked with a sauce and absorbs flavor well. Making seitan and gluten will open up a new horizon for you in the world of vegetarian cooking. Try substituting it for animal products in former favorite recipes or those of non-vegetarian friends and relatives. Then get your creative juices flowing and experiment when making seitan by varying the flavorings and cooking methods.