I have noticed some fantastic images using fog and haze on the forums. I take it that such work is done in Photoshop after the image has been rendered?

In this regard, any recommended tutorials for postwork in Photoshop?

It can be done in Poser, too.  Fog and mist can be simulated with Atmosphere.

Offhand I can't remember how to use it - it takes so long to render I rarely use it - but I'm sure there are plenty of others who can point you in the right direction.

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I forgot to add you can also fake it in Poser with a prop.  Set a square to transparent and throw a transmap on it, depicting clouds or mist.  You can stack up the squares to simulate increasing density.

Here's an old, old pic of mine using the prop technique.  A curved prop for the background and a plane for the floor.  These were both part of a pack by cmdctrlesc but they are easy enough to make in Poser.

I may just do a retake of this pic one day. :)

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If you're lazy like me [LOL], Jepe has a great set available at DAZ for use in Poser for easily placed fog, steam, smoke etc. 
I think it's called Steamz!

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At RDNA ,you can find Rain and fog meshes-materials with pose for using in Poser. Also nice one for Poser :Weird Weather by Cyllan (i bought it in Content paradise i think)
You can also use some brushes in Photoshop in postwork (buy it or create it)

If you want to add mist smoke etc in postwork, there are a number of cloud, smoke brushes sold at Rendosity for Photoshop that do a great job.