I tend to take a rational approach and not bother with amended returns when it is simply a timing difference (from one year to another, with no change in the amount of the loss.) I am not certain as to the strict legalities of when the stock actually became worthless, but as a part time tax-preparer, I would certainly entertain a taxpayer's thoughts that starting the bankruptcy process is grounds for claiming the worthlessness, so I don't believe there is a fixed line and you are within reason in your original claim.
