Is there some optimal depth for the prop?

There is no absolute rule. The main thing is keep it low enough, so it does not blowout, when running the motor

As a starting point the top of the motor should be about 12 inches below surface.

I move it around depending on conditions.

If I am fishing in shallow water, I move it up to to avoid snags and bottom. You just don't want go too high or it will blowout.

If I am in rough water I move it down.  to keep it in the water

Mostly depends on the waters you fish.  Heavy boat traffic with big wakes or big water under windy, white cap conditions requires the motor be two or three feet deep.  When the bow is pitching up and down under those conditions, if it's not deep enough, it stays out of the water more than in and makes it almost impossible to control the boat.   But there is a bad side to having a motor that goes that deep.  When fishing calmer, shallow water, you will want the motor up to about foot or so deep, then you have the head of the motor sticking way up and tends to get in the way.  

When transom mounted on a jon boat, I want it deep enough I can spin the motor 360 degrees without the prop hitting the bottom of the boat.  

On a bow mount, if I rarely, almost never fish rough water, I try usually want one that the head is not more than a foot up, so I do have the option of dropping it down some if the water gets a little rough.

 

As high as it can be without blow outs.  Where I like most to fish this means running slowly barely below the surface so it doesn't collect weeds.  When I have to fish deep it may need to be way deeper dependent on the waves/wind that day.

Ever-changing (dare I say, dynamic?)