There is no perfect way, all are a compromise.
If you mount them parallel to the water, horizontal, they will work the best of any position when the boat is going in a straight line. However, if the boat is cornering, or in quartering seas, they will raise and toss the stern of the boat in the opposite direction of the way you want to control the boat when deployed.
If you mount them conventional, they work the best when the boat is cornering, actually laying on one side of the Vee for hard cornering (mostly in race conditions where you really lay the boat on its side), because then they are parallel to the water and do not raise and toss the stern of the boat in the wrong direction.
Most of us doing new boats now, especially if they are going to be used in all types of conditions, split the difference. Best compromise for handling in most positions you may get yourself into running hard. (This comes from Fountain factory race boat testing and SVL race testing).
Just think about what the tab is doing when it hits the water. It raises the stern of the boat to keep the bow from skying. So, for the best control, it need to be parallel to the water it is using to raise the stern. Anything else it will want to do odd things to the boats attitude.
The compromise.
