Just beware of that bloodey TAT gage guys! Make sure its reading what it should read. Ive had erroneous TAT indicating +17 at FL290 at M.75! ALWAYS keep an eye on the underside of your windscreen wiper as a backup, and a reasonable knowledge of expected TAT at various levels at your usual flying speeds. If in doubt throw open the valves and be done with it. Most modern engines can handle engine thermal anti-icing up to a very high ambient temp (+38C in 737 CFMs) so its not like youll burn anything.