The rule of thumb that I've heard is if the engine can be off for 15 seconds or more then it is likely better to turn it off. The one time that I coast a long time but leave the engine on is if the engine is still cold -- you need to bring it up to about 165F in my car to get it off of the richer mixture, so idling while coasting to warm it up seems worth it.
And bump starting is probably better than using the starter, so if you have a standard, you can turn it off, wait a second and then turn it on again so that you can bump start it as quickly as possible, as needed.
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