In the endgame, when your Ogre's speed is down, you no longer control the pace of the game. But this doesn't mean you must ignore enemy units. The slower you're going, the more turns each enemy unit has to put further damage on you, and the more advantage you can get by destroying it early. Once it's destroyed, the Ogre can make its way to the CP in peace. If the CP is the first target, the howitzer may have enough time to destroy the Ogre totally.
Some players believe in keeping a few missiles back as a threat.
I don't think that's a good idea. Either fire it at a low-defense unit like a CP or HWZ, guaranteeing a kill, or hit a unit in the Ogre's line of march, so disabled units can be finished off next turn with gunfire or ramming. Target choice must be ruled by tactical position. If several units are advancing from within a howitzer umbrella, you may want to salvo all your missiles at once and fry the closest half-dozen. If, on the other hand, the enemy units are coming a few at a time, you don't need to use your missiles as soon.