If I know that I'm going to try to compromise a system and access its username/password lists, is there an advantage to creating a number of accounts to which I know the password prior to the break-in?

Does this make it easier to break the other accounts once I have access to them encrypted? As in, I know that the account with username X has an unencrypted password Y, so now I have guideposts to tell if my cracking attempts are pointing in the right direction, trying to get back to password Y from the hashes. I imagine there would be something of an advantage to already knowing, say, 10,000 plaintext-encrypted pairs in a big list.

If this is the case, should one be concerned in managing a system that sees a dramatic uptick in new user registrations as a precursor to an attack?