Maybe I'm just not thinking correctly, but I don't see why people would "hold" a balance in a pool that charges fees as a percentage longer than a day anyway. In the case of hashcows and converting directly to BTC, it's 2.5% fee. So regardless of whether "hold out" your auto-pay to cash out @ 100 BTC, or 0.01 BTC the percentage of fees to payout is the same. I don't see any benefit to holding any BTC in the pool account.

On pools that charge a flat fee per payout, I might could understand as fewer payouts would mean less fees taken from your mining efforts...