Ski - A slight modification on what Searry said. Assuming you're going to launch your invasion of mainland Italy from multiple ports including Bone (size 4), and Tunis, Algiers, and Oran (size 6), send at least one of the TF's to Bone immediately after the invasion of Sicily so that the units there can start prepping for the Italian invasion immediately. You only get as many invasion prep points per turn as the size of the port, so you want to send the TF's to the smaller ports first so those units get a head start.
Also, you should leave the TF's at the beaches of the invaded hexes that aren't cities, and first withdraw the TF's from the hexes that invaded cities. That's because follow-up units can land in the former hexes and immediately unload, whereas follow-up units landing in cities with damaged ports can't unload the same turn, and if they're larger units (e.g., armies, armored divisions), may not be able to unload until the port is fully repaired, which will take several turns.
Finally, taking all the above into account, withdraw the TF's with the most damage first, so they can repair while also prepping for your next invasion.
But as Ski noted, the task forces make excellent mobile artillery units against Axis units on the coast. You may even be able to turn off air support during those attacks because you don't need it, then turn it back on afterward (don't forget!).