Another 2 cents for you, Foz...
All of the above is true, but in RL it would also not be a good idea to leave the carb heat on in a 150/152 (which I know you fly often in the sim) during takeoff! That's when the difference in power output may be critical. Even more so, of course, with an even smaller engine.
If there's ice in the carb as you begin your takeoff roll, in a 150, you will know right away (low tach reading or no increase in rpm), and you should be able to abort safely. To me, that mindset is more valuable for safety than planning to use carb heat on the takeoff roll and climbout. A 150 or 152 needs all the hp it can generate for takeoff.
If conditions are so bad as to require carb heat just to prevent icing while preparing to take off, you might want to cancel your flight.
That being said, sometimes carb heat can be used while warming up in very cold weather (remembering about the air filter)... not so much to prevent ice as to just help warm up the engine.
A word about conditions that my require carb heat: some days it's obvious, some it's not so obvious. Only time I ever picked up carb ice (in a 172), it was very cold out, but it was a pretty dry day- it took me by surprise. What was most surprising was that I was at a cruise power setting... it's "not supposed to happen" when you have the tach well within the green arc. I guess in that instance, the cold made up for the relatively low humidity level. But even in warm weather, with no clouds visible, it can get you.
So... what does this actually mean? Takeoff aside, can you leave it on? I never have. Pitot heat, maybe, but not carb heat. My reasoning is that the most dangerous time to get ice is during the approach, when you are getting lower. It takes a few tense moments to clear carb ice... including the brief period when the power drops off
even more when you turn on the heat. In cruise, it's alarming but no big deal (I was able to hold my altitude that one time, and it cleared up very quickly), but at pattern altitudes and airspeeds it could be a very bad deal. So the normal method of making carb heat part of the pre-landing checklist ("abeam the numbers" or whatever) is sensible. You might also want to turn it on if you are throttling way back to descend from cruise, as a precaution (it would suck to restore power as you level off from your descent, only to find there
is no power).
Also, unfiltered air may not be so good even while aloft... or the carb heat door mechanism may get stuck... etc... I only use it pre-emptively when I think I'll really need it -which is whenever I throttle back to the bottom of the green arc, or beyond. and of course, if I do pick up ice in cruise, I'll leave it on!!
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