Jesus. This makes it reasonable to just buy $100 worth of your own game every month, just to make sure. Assuming that the number of real sales cover Valve's percentage and then some. Yeah, that's a non-zero opportunity cost for you, and additional float for Valve, however petty it may be. But for a small developer, maybe that makes sense.
The break even point would be at a balance of 23.08$. However, if the account balance doesn't expire, buying your own game to put you over the threshold would be checking the couch cushions for loose change level of desperation.
Oh, it's petty cash to be sure. If you have $100-ish bucks to throw around, you probably aren't going to miss much by not doing this. Unless, of course, letting someone else take even one dollar from you in this way is against your religion or something (i.e. the principle of the thing). Conversely, if you need the handful of dollars this makes, you probably don't have that kind of walking-around money in the first place.