Generally depends on the stuff used.
I usually use HD stocks.
Also there's the component of not making it low quality yourself.
You never want to resize something so much that it's distorted.
If you're not using a stock or render, normally sharpening works but not over-sharpening.
Can also get Topaz clean or adjust, which are fine, as long as you don't use them for the "cool Topaz effect".