I almost always keep the vents to draw fresh air and blow it on the windshield, with or without the fan. For most of the year, I don't run any HVAC and leave my knobs, from top to bottom, on the green dot, fan off, vents to windshield with recirculator off. When I do run HVAC, it is for one of three reasons:
1. Windows fogging up. Usually only an issue if the car was parked in a warm garage then taken out to the cold or I had the recirculator on due to bad air quality outside (smoke in a valley, coal-roller in front of me, sitting in traffic, or going through a long tunnel). Usually just turning the fan on one click with fresh air or cracking one of the rear windows is enough.
2. Cooling the interior. Usually only on very hot days, as even having the windows down in 85 F weather keeps me cool.
3. Heating the interior. Again, pretty limited usage, usually only when outside temperatures are below 15 F. We got spoiled by 50 F temps nearly all the way through December, so I'm not acclimated this year and have been running the heater much more often. I usually start out with full heat and high fan speed (max on the coldest mornings like today at 6 F). I then drop the temperature down a click or two from the top and drop the fan down two notches. I keep some air going to the dash vents to keep my hands warm (heated steering wheel?), and send the rest to the floor while cycling the recirculator on and off based on window fog. About two miles from work, I drop the fan and temperature to one click above off and green dot and all air to the dash to use up the stored heat in the coolant. Once I see the heater kick back in or I arrive, I turn the fan off, temperature back to the green dot, and send all air to the windshield.
I've experimented with running heat and AC at the same time, but I'm not convinced that the AC runs at all, let alone making any difference. Also, I don't believe that the ductwork allows mixing heat and AC.
