I agree with the other posters... 8 is too young for any "situation" that may come up. No matter how much you tell her not to answer the phone or door, if she sees someone she knows, she may answer anyway (and we know that statistically, most child molesters are people kids know). Also, people are bound to find out that she's home alone, whether they see her walking home by herself, or just hear from a friend of a friend.
In addition, you never know what might happen. As an adult, I've had situations that I've had to react to, that a child might not know what to do (house was hit by lightening, water pipe broke, light switch shorted out and started smoking, neighbor hit and demolished brick mailbox, etc.) - all things that could happen to your daughter while she was home alone.
As the other posters mentioned, chances are that 99% of the time things would be fine, but you can never foresee what might go wrong and prepare her or every contingency. At 8 they simply don't know how to make those quick, emergency decisions (that adults sometimes struggle with). I'd wait until she was 12.