I would say keep them within 10-15 miles of your start point. With no more then 1-2 miles between clients. If its a bigger client it could be 10 miles from your other clients, but only if its big enough to be worth it.
Thats kinda how I figure it. At first I was thinking I would go anywhere within 20 miles, but when it came down to it, and I got some calls 20 miles away I passed on them, just not worth it. Remember you have to figure in your drive time when you figure how much your making an hour, so if your making nothing driving half an hour, then $75 for the half hour your there, then nothing for the half hour drive back, well you only made $75 in 1.5 hours, and you had the risk of getting stuck or stranded a long ways from home/shop. The people on here that keep thier routes tight are the most efficient, and are the ones making bank.
There are exceptions though, I would take a job that was 20 miles out, if it could be done when I wanted to- like a day or two later- when I had nothing else to do.