To determine whether you should freeze a casserole before or after you cook it, first ask yourself if it is likely to get dry (or less appealing) from overcooking. If the answer is yes, you are better off freezing it first, thawing to room temperature, and then baking. That way you don't risk that it will lose its appeal when re-heated.
When a casserole has liquid, sauce, or lots of creamy, liquid ingredients, chances are you can cook it first, then freeze. Think of pot pies, lasagna and turkey tetrazzini. For more tips on freezing recipes, see our Fill-Up-Your Freezer Storage Guide.