Unfortunately that strategy only works if you suspect you're about to be obliviated. Which makes it mostly ineffective except against people who play fair and ask before mucking with your memories.
Or people like canon!Lockhart, who bragged but didn't ask, though that might be a petty distinction.
Or like it was first revealed in chapter 6, as stated above, or again in chapter 17, when one is in a situation wherein there's a high likelihood of Obliviation. Honestly, unless someone springs out and sneak-attack Obliviates you, you'd probably have an inkling that someone trying to keep their secrets is either about to kill or Obliviate you, which should be enough time to chew your cheek or lip.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: