No that's perfectly fine. If you're gunning a moving hog you can always shoot anything you like, except teammates.
If your team has the lead, sometimes it's good strategy not to shoot a lone hog. By killing them, they may respawn ahead and then nade your hog as you pass by.
When the server is nearly empty, when a 5th person joins, a rocket hog will spawn. If your team has 3 people, and the other team only 2, and you guys get the rocket hog, you might also not want to kill too many enemies since you have an extra person AND the rocket hog.
But it's always within the rules to use the gun. When we tell people not to slay, we mean people who are walking around nading when they could have been in a hog.