With all respect to 9/11 things have returned back to normal and although there might be a small risk, access to the flight deck becomes easier nowadays... (of course not in the US...)
There are several ways of allowing access with no risk at all. I remember a Capt. on a transatlantic flight who, after we asked, invited us to the front, and chatted with us about 30 min. in front of a locked cockpit door. That's a pretty neat way getting to know the guy you're allowing access.
Remember, final decision lies with the Capt.