I've used the commute excuse as well. I actually just interviewed with a family that lives 40 minutes away, and with 3 kids they only wanted to pay $10 an hour. So instead of saying it was the pay, I just said the commute was too long. But I guess saying pay is an appropriate excuse as well, although I fear it makes me come off as greedy and only caring about the money. But I got to be able to pay my bills.
With a family I don't click with and can tell their personality is going to be one thats difficult to work with, I usually end up telling a lie and saying I accepted another position. I feel badly lying, but there's really no good excuse when you live close by. You can't nicely say that you just don't think you'd end up liking them.