I highly recommend not changing your bids daily, for a couple reasons. You need at least a couple weeks worth of data to really show any significant patterns or to see what kind of effects your Average CPC has on your business. Adjusting your bids daily only dilutes your data.
Also, don't get greedy. You might be getting $1.00 CPCs with average 100 clicks/day [You spend 100.00 to make 150.00]. 150% ROI. Profit = $50.00.
Drop it 10% to $0.90 and ROI remains constant but if overall clicks plummet to 80/day, you're losing money [you spend $90.00 to make $108]. 180% ROI. Profit = $18.00.
Even though you're ROI has increased 30% from 150% to 180%, you're really losing money because of the decrease in clicks you're receiving ($50.00 profit vs $18.00 profit). The real key is keeping your traffic/clicks constant and decreasing your bid, which is almost more of an art than a science.