I've had a bad experience with Kodak online printing service - I had a free prints coupon so I decided to use it and order some personal 4x6 prints. Paper quality was horrible (I could have done better buying my own paper and printing on my Canon MP160) and colors were blown out even though my whole setup is calibrated regularly using a colorimeter and color profiles. They use their Perfect Touch technology which pretty much alters the photo the way
they think it should be - call it for what it is - Perfect Mangle.
I agree with Lowell, with one caveat - if those proofs will be shown to the "customer" I would do a few test prints to make sure they look good enough to show. I have found that most big box stores like Target, WalMarts, Costo, BJs, etc are likely to have either poorly run machines or poor operators, so your print quality will be hit or miss. They're fine for birthday party pics, but not if you're trying to impress a bride and groom about your skill.
There are exceptions though, which is why you need to try them out ahead of time. The WalMart 10 miles away from my home actually has a beautiful Fuji Frontier (700 series i think) and use Fuji papers that produces some absolutely gorgeous prints with vibrant colors, perfect for nature shots or last minute short orders. As little laker pointed out, any problems and a good lab tech can help you out right away, so scope them out too - find the ones that really know their stuff!
Oh, stay away from the Kodak dye sub printers and glossy shots - in my opinion glossy prints cheapen the shot anyway
For my pro printing, I do 99% of it at WHCC, but would just as easily consider Mpix or Meridian Pro. I order prints and they're at my door in 2-3 days, usually at less of a cost than going to a local outfit, even WalMart.