When Mystic Tomato is destroyed or any card that got its effect activated when it's destroyed by battle, its effect will trigger when it leaves the field meaning at the End Step, NOT DURING THE DAMAGE STEP.
A monster is destroyed during the damage step but only leaves the field during the end step.
If a monster's effect is: "When this card is destroyed by battle and sent to the graveyard" it means that its effect will only trigger during the end step AND if it's sent to the graveyard. If it's only "destroyed by battle", its effect will trigger whether it's banished or sent to the grave.
In the case of Zenmaines, Maestroke or Leadblow, it would either mean that it's a bug OR that their effects trigger during the end step. In that case whether it's mandatory or not, the effect will trigger unless it miss timing.
As for what Steeleagel said, please read more carefully the example used in the wikia:http://yugioh.wikia.com/wiki/King_Tiger_Wanghu
Wanghu's eff can only trigger if the opponent manages to summon thanks to an eff during the damage step which starts a chain
. In that case, considering it's a chain, it doesn't matter what happens because anything can be chained to an effect (speed spell 2 or 3). Considering Wanghu's effect is obviously speed spell 2, that example is quite bad.
The only exception in the damage step with monster effects are flip monsters because their effects trigger at the end of the damage step.