This is a good point. Something that I believe was brought up recently with Buffy -> Whedon. If you now look at Whedon differently, and then go back and watch Buffy, its very likely you see things differently, beyond just the scope of time, shifting cultural norms, and so on.
On the other side, I can still look at the Cthulhu Mythos, look at it not as a racist work, but as one which speaks to the insignificance of humanity, the absurdity of our existence, and it has nothing to do with Lovecraft's racist views and era.
Ultimately, its a judgment call each and every time.