What makes me confused is that sevral commanders have the oppinion that bleed air flow should be MAX until desired diff press is obtained.
Indeed, that's correct. This will then provide you with sufficient air mass to maintain a cabin descent rate. What your Commander is trying to establish is a cabin altitude that's similar to the airfield elevation prior to selecting air flow to MIN.
I'm not familiar with the DHC 8, but if the Pressure Controller doesn't have a 'barometric pressure setting capability' i.e. that it's referenced to 1013.2 mb, then unless you do some mental arithmetic to take into consideration the 'actual' QNH then you should set the desired cabin altitude to airfield elevation plus 500 feet. The reason for this quite simple; if the QNH (on the day) is very low and that you have set airfield elevation into the Pressure Controller Cabin Altitude Selector you WILL land with differential pressure remaining.
This might well be the reason why you 'sometimes' land with Diff Pressure remaining! Therefore, 'compare' QNH to 1013 mbs i.e. if the QNH is lower than 1013 (32 feet per millibar); add that difference to the airfield elevation and set it on the Pressure Controller.
If that's too confusing (or you're busy with other things)... don't do it; just set the Pressure Controller to airfield elevation plus 500 feet.
I hope this helps.
TCF