Msg#: 4152645 posted 1:16 pm on Jun 15, 2010 (gmt 0)
as you bid more and your position goes up, your ad's ctr will increase - at the top of the page, it's gets more notice. but G corrects for positional effects on ctr, so your quality score won't go up just because your higher position's ctr went up, so the higher bid will reduce your ROI / ROAS. so you've got to decide if you want to chase more volume at the cost of lesser unit margin.
likely the best method is to optimize your ROI / ROAS, not the other metrics. see this G video for more on doing that:
Google AdWords Bidding Tutorial with Hal Varian, G's chief economist [youtube.com...]