When stopped and seated, your feet should touch the ground, but not your heels. You should be able to balance the bike while standing still and seated in the saddle. Experiment with what feels comfortable. This is only practical on what may be extended climbs or downhills, as you need to dismount to do it. This is personal preference.
Some riders go for the least weight and a skinny saddle.
This may be good for racing, but hard on your butt for rocky and bumpy riding, unless you have suspension. Adjust the tilt and forward-aft for your comfort, and experiment a little with this. Usually the height of the seat should be set so that your leg is almost fully extended at the bottom of your pedal stroke. Also, in difficult terrain you want to be able to reach the ground with both feet and still keep the bike upright. Sometimes we find it helpfull to raise the seat slightly going uphill, and lower it slightly going downhill.