As with your seat height adjustment you should learn what seat position your body likes and then use it whenever you have to pedal for any significant time on any bike.
When setting seat position, the most important thing to consider is your comfort. With that in mind use the following guidelines to steer you in the right direction.
The horizontal position should be set so that when your pedal is at the very bottom of its stroke, the front of your kneecap is directly above the pedal axle.
The seat angle should be set so the seat is generally level. Beyond that your comfort should lead the way. You should feel like the bones in your rear end are doing most of the support work but you shouldn't feel like the seat is trying to push you forward or rearward.
If you are experiencing issues with numbness in the crotch area while you are riding, there is a good chance that a change in your seat position can help fix the problem. This can be greatly effected by the seat itself, but as far as position goes, adding a little forward angle may help.