I have trouble believing vast majority of miners are not selling for fiat immediately, because if a miner is not converting to fiat, then he's just paying facility/electricity/equipment cost out of pocket, then he's essentially buying BTC with fiat. He could have just bought BTC on an exchange if he wanted to buy BTC with fiat.
I think they are selling to cover their costs, sure, but I don't believe they are selling every bitcoin they make.