It's complicated.
The simple answer is that normally a support hits creeps to push the lane. When you do this is somewhat nuanced, but essentially you frequently push at the beginning if you want to hit the early 2 or 3 (which is NOT always - don't fall into that trap), or when you push the enemy out of lane and you want to push up to the tower before going back to base or going for dragon, or even to take the tower if it's low enough or you have Jinx. Pushing at the wrong time opens you up to ganks, and gives you less running room and them more. In such a case, you want to only last hit, ideally after the last minion attack starts but before it hits, so you have the smallest effect on the lane. This means only the marksman should be hitting.
The only other time I can think of to hit creeps is to help your marksman under tower, but if both you and the marksman aren't fairly good at this you will do more harm than good. I usually don't try unless I've noticed the marksman is very good at farming. Kalista provides an exception: if you "click" well with one and have a lot of practice supporting her, you can help farm very effectively, since her passive causes the minion gold to go to her.
But, also, when you DO attack minions, you still don't want to make it impossible for the marksman to farm. Try to make sure there are never two minions at "kill range" at once, since the marksman can't get them both that way. This will take practice. Sometimes, though, inexperienced marksmen will get lost and miss more when you do it perfectly than if you didn't do it at all. Usually though, the benefits of pushing at the right time outweigh a couple lost farm. I see this most frequently in solo-laners who are pushed into the marksman role, since they're not used to taking into account another champion's damage. Pushing at the wrong time AND causing the marksman to lose farm, however...
That being said, it's probably better to err on the side of caution. Doing it at the wrong time in the wrong way is usually worse than not doing it at all - though not always.
