As I understand it, there are two penalties to using carb heat: maximum power is reduced, and in most cases the induction air is unfiltered. In most cases these are not relevant at the cruise. As far as I can work out if you lean correctly there should b no increase in fuel consumption, and while in the cruise at low level full power is not required. Therefore it is only above full-throttle height for required cruising power that there is any penalty for using constant carb heat in the cruise.
However, the only aircraft in which I have ever suffered carb icing in the cruise is the Cherokee 140, and then only in poor visibility, so on a clear day, keep cold and just check during FREDA checks.
As for the final approach, I personally think it is unimportant. Icing below 300' on approach is neither likely nor catastrophic (should be within glide range by the time it could stop the engine), and on the go around you should be checking carb heat anyway.