I think that if you are cooking indirect on a kettle and not using any wood for smoke it is OK to control the heat with the top vent, but if you are smoking I would use the bottom so that the smoke can exit freely.
2 OTG One red one black
22 1/2" black Bar-B-Kettle with rotisserie ring
Brinkman dual zone charcoal grill
Webber Silver Genises B
3 burner camp stove