If you want your whole turkey to look it's best on the platter, trussing the legs is the way to go.  By trussing the legs, the bird will have a more compact shape and it won't spread out when roasting. Use kitchen twine, unflavored dental floss, 100% cotton thread, or fishing line to tie the ends of the legs together.  Most chefs choose to cross the legs before tying, others place them side by side. If you choose not to truss the legs, it will not affect the taste.  Chefs who choose not to say the leg meat cooks quicker as the air circulation is better.