Where does rounding come into it?
By rounding I meant that fact, that you get paid in "chunks" (couldn't be otherwise). So creating an address is effectively not totally free, you'd be about -0.34 BTC on average just by having an extra address.
Sorry if I wasn't clear.
Correct me if I'm wrong.
You aren't paid until you reach the threshold, and then you're only paid for what you mined up to that point. So, at any instant other than when you've just been paid, you're overdue an amount X, that's between 0 and the threshold (can be a bit more but let's forget about that). Statistically, you're overdue half the amount of the threshold on average, multiplied by the number of addresses.
While it is true that if you split your mining between multiple addresses you will have to mine longer to reach the payout threshold. But even if you never reach the threshold, auto-payouts kick-in after a week of inactivity so that only means you'll get payed later, not less.
Ok. That will continue, right?