I don't buy them back, never. I just mine them. My question is always whether I should hold on to the coins I mined because they have the potential to go to $100, or whether I should sell them before they go to $0.
If you're just mining, and not speculating at all, then just sell them as you get them.
Impossible for small miners. Say you are selling 5 BTC at $70 bucks and want a wire transfer.
You go on tradehill, pay fees just to sell the coins, then pay a $45 withdrawal fee to actually get the cash out.
You end up getting 25 bucks for 5 BTC, or $5 each. Maybe even less if your bank has fees for receiving wires, or ATM withdrawals etc., making your mining efforts essentially worthless.