I want to ensure my pages will look right as soon as IE9 comes out.
For example, I have already noticed that the background-size in the IE9 platform preview works, but is inconsistent with chrome, chrome resizes maintaining the aspect ratio, while the IE9 platform preview distorts the aspect ratio.
But who knows if this will be addressed or not by the time IE9 is released.
So should I just wait until IE9 is released before I start addressing anything?
You should start testing your pages now. There's the possibility that you could report a bug and get it fixed.
I wouldn't be testing against the Platform Previews that they've released thus far.
They're nowhere near even being feature complete and your page might work in one revision and be completely broken in another.
Wait until there is a Beta released for developers to test against before you worry about anything.
Conventional Wisdom should tell you that you should test as soon as possible on as many browser as possible as soon as possible, but there should be a finite line where bugs/issues are your own or IE's.
Test, and quickly check the validity and report them to Microsoft as well.
I would not put them in high priority on your side until IE9 get nearer to release (or late beta).
It is a good practice you check in almost any platform what you expect your app might run in future (Until you will not bypass your deadline).
Personal preference, but I've waited. In the past I've tried to get a jump on new versions of IE, but it's always seemed to be a waste of time. Who knows though? Perhaps this time will be different...
As a side note, it most likely depends on your audience. My audience is slow to adopt "new" technologies. But if your audience is akin to the SO crowd, whom I'd expect to contain a large number of early-adopters...