-snip-
But here in case if my account is hacked, the person may edit my post where my bitcoin address is present and I wont have any access to my PMs then. So in this case, I will have to note the post ID and the PM ID to be on a safe side?
That is correct and theymos said in the past that the best way to prevent this is to send a PM with a signed message to DefaultTrust and write down the PM ID. You could even delete the PM afterwards so a hacker can not see it. It is however still in the system and theymos can access it as he also controlls the DefaulTrust account, but never deletes PM send there.
Another way would be in an auction thread, since they can not be edited and not deleted.
What Gisado suggests might also work, but it would give someone else (the person quoting) power to manipulate.