Basically, toms gobble for 3 reasons:

1) Attract hens

2) Exert dominance

3) "Shock" gobbles...vocal responses to unexpected, loud, foreign, or irritating sounds.

Anyway, to answer your question. I've had gobblers sound off at midnight to car horns, owl hoots, coyote yips, etcetera. I've had gobblers sound off flying up to the roost. I've also been surrounded by multiple toms, and could not get them to shock gobble at all times...and probably a grenade blast wouldn't stimulate them to reveal their location.

Personally, I prefer to owl hoot in the morning, well before light...say ~4am. I prefer to let them feel comfortable when they go to roost. If I do call at night, I find that ~2hrs after sunset, which is about 1 hr after nautical twilight (when the horizon is not visible), tends to work the best. And actually, I find that if I call right after flyup a soft hen cluck usually gets the best result...sunset is a little early for owl activity.

S&R