Greg,

When you mix up the one liter packet with one liter of water, that is called the stock solution. When you see times for 1:1 dilution, it means you mix 1 part stock solution with 1 part water. For a single roll of 35mm film that would be 4 oz of stock solution and 4 oz of water, and you would dilute it at the time of use. This is usually what I do, but a lot of people use it undiluted and a few people dilute it even more than 1:1. Any time you change the dilution, the times will need to change too, but the charts usually tell times for both undiluted and 1:1.

Diluting the stock solution gives you longer times which will usually help you be more consistent. Anything less than about 6 minutes makes it hard to control. That's the biggest reason I do it, but as you mentioned, it also saves money. And, as Aaron mentioned the more you dilute D76, the more grain and sharpness you get. That can be good or bad depending on the film your shooting and how you want it to look. But, the difference between straight stock solution and 1:1 dilution is nothing to worry about. It'll look good either way, so give it a try.

Paul