I think the others have pretty much summed up using take 10 and 20 for skill craft checks. You wouldn't be able to take 20 on a profession check since the check is made once a week and reflects your income for that periiod of time, taking 20 usuall translates into taking 20 times as long so for a single week's worth of income you would be taking 20 weeks in order to accomplish this. You also cannot retry a profession check while you can retry a craft check, so the premise of taking 20 doesn't translate very well - pretty much you ar making 2o different attempts and using a law of averages to get a 20 once during those rolls.