I'm all for thinking on your feet if needed, but why make life harder than it needs to be? Like I mentioned earlier, we have about 200 songs we draw on, and we usually play 6 different songs per week, but the BL sends an email by Tuesday of the setlist for the coming Sunday so we can all practice them. Granted I'm not doing any serious woodshedding, but I like to run through everything a time or 2, and clean up any trouble areas if there is a difficult part. Of course on occassion we'll show up and the pastor will ask us for a last minute change/addition, which we accomodate, but there's no reason to leave the whole setlist unplanned.