Yes, yes, yes, xeylode, you are absolutely right about continuing to damage your opponent until an opportunity to kill comes up. That's how Samus plays from what I've seen. I was actually thinking about the use of fair recently. Its a fantastic killing move, as well as a fantastic spacing move. In my opinion, if youre going to use it as an approach, then is best to actually let your opponent power shield it so that you can follow up with double jab, turnip, jab to grab, or whatever you want to do. Unless I've been doing this completely wrong, then if your fair is shielded, then it doesn't get decayed. If it is shielded, then even though it is decayed, you still get that extra damage, which is always a good thing.
Fair is a great move, but its better of you don't use it too much for damaging. I always stress to use nair, bair, fair, jab, turnips, grabs, etc for damaging, but a lot of people can't help using fair, including me. I actually use it a lot more than I should. But, peach does have other killing moves.
Like you said, you shouldn't stop attempting to damage your opponent until you get a surefire chance to kill them. Fair is great for killing, but if you keep damaging them, then they eventually have a high enough percent for half your moves to kill.
I love fair, and I use it a lot, but a lot of the time, I actually hope that my fairs that are shielded, so that I can still do damage with other moves while saving my fair.