>never realised what Selective Gaussian Blur was really useful for

This is probably the filter I use most - basically it smooths colours that are close positionally and colourwise to avoid graininess. A good way to clean up images quickly though it can look unnatural if done too much.

I often use a little SGB and then a little Unsharp Mask when touching up images for low-res or online output.

I'm not professionally trained but do use this for commercial purposes and have for some time.