That depends upon how you use pitot heat. Presumably you turn it on is visible moisture at or below 0°C, but you may turn it on at any time in cloud, or leave it on throughout the flight.
If there is no drop in manifold pressure, there is no need for alternate air. Carburettor heat is used as an anti-ice feature during decents, in order to prevent mainly fuel icing. (Fuel ice forms downstream from the point where fuel is introduced in the induction system. The temperature drop due to the vapourization of the fuel can condense moisture from even clear air, then allow that water to freeze in the induction system.) This is unecessary in fuel injected engines, although fuel injected engines may still suffer from throttle ice at temperatures below 2°C, if the throttle hasn't been located in a warm position within the cowl.
Fuel injected engines (which are fitted with alternate air) may suffer from impact ice - formed by cold water droplets hitting a surface cooled below 0°C. You need visible moisture (i.e. cloud) for this to occur, and ice builds up on components like the air scoop. It is noticeable by a drop in manifold pressure - but as the alternate air source will be unaffected, then changing to alt air will fix the problem. Unlike Carb. heat (where you are trying to prevent ice) there is no need to have it operating before the problem occurs as you are circumventing the problem. Indeed, operating alt air all the time in cloud may hide a development of ice that you really want to know about. I wouldn't use it unless I had to.
During a ground test there should be virtually no drop in MP.
[ 07 September 2001: Message edited by: Checkboard ]