Ideally, you want to put the meat in the smoker, set the temp, and let it go until the meat is done (usually by reaching a desired IT). However, there are times when you might need to open the door, you just don't want to be doing it too often. I would also recommend not opening the door for at the least the first heating cycle when the wood could catch fire with the rush of oxygen. After the first hour, if you need to open the door, be a quick as possible! The unit recovers quickly to the desired heat setting, but the longer the door is open, the more heat you lose.
When I do BB ribs, for example, I leave the door closed for the first 4-4.5 hours. I then open the door to remove and add BBQ sauce to the ribs, and then put them back in for another hour or so. What I try to do is plan opening the door when my #2 is at the maximum temperature swing, so while I will lose some heat, the unit will stay close to where I want the temp. For ribs, I set at 225-230, but with the temperature swings of +/-15F, I would wait until the temp was at the higher end (say 245F), and then open the door. By the time I get the ribs out and the door shut, the temp might drop to say 210F, but the then the unit will start heating again to bring the temp back up.
In all of the smokes that I have done, I can't think of any where I needed to open the door more than two-three times for the duration. So again, I recommend only opening the door if it is necessary...avoid peeking where possible!