The general attitude I have experienced is that Flight Deck crew are 99.99% of the time enthralled to invite you in after the flight. The ones I have spoken to love it, because it gives them the rare opportuntity to show people around.
The only problems I have are with tight turnarounds: unless the crew are really nice, they just cannot fit you in on a 20 minute turnaround.
The second problem is cabin crew who share the same attitude as captainbritboy, and think that post 9/11 any visit to the flight deck is against every rule in the book. If that happens try and persuade them to ask the flight deck crew, who will most likely prove them wrong!