You Need to be using a quality proxy service to ensure you dont get blocked, and you dont want to be stuck getting your results as local results instead of national, etc.
I highly recommend
http://www.yourprivateproxy.com/669.html, I use the 10 proxy pack and run the system for thousands of queries per day, You will still need multiple API keys for Bing and Yahoo with the proxies as they will identify you by your key.
But with the proxies you can avoid using the keys in the first place, and scrape all day long.