Where it's possible to add such info - most of the time I give credit to both freebie and commerical products.

Note the both there - because the prices for poser content often doesn't reflect the amount of time and effort put into something. While it would be nice to pay the artists more, I would find hard to justify paying $70 for a mesh or character, when thats around a weeks shopping.   

Also with the rare bit of commercial work - say in a magazine - you don't have a large area of space. One line tops at the most and thats assuming they even credit you properly or at all.
If i get a photo in my local paper it will say "contributed picture"   

Theres one thing here that hasn't been dicussed = all of this assumes you actually read the readme and licence file. Unless you have a problem with the model who really does that this every time ? 

Pinky - you left the lens cap of your mind on again.