If you want to have a look after the flight, make sure you're near the front of the A/C and get in there straight away. If you hang around in the back of the cabin waiting for everybody to get off before speaking to the crew it won't work because you would affect the turnaround. I've seen this happen and I have had to disappoint people who wanted to have a look in the cockpit for that reason.
Disembarking normally takes a few minutes. This is really the only window there is for a flight deck visit. See if you can speak to one of the pilots before the flight, ask if you can pop in after the flight. Most of us are happy to oblige, but don't take it personal if the answer is no. On the day there can be many reasons why there is no time for a chat.