I think it's trial and error really. I think Loc and the rest of the guys all have different methods. None of wich are incorrect but just a matter of personal taste. I returned my prof software to the re-seller after a very long 3 week fight with them, and have decided to use Blender. The UVmapping is fairly straight forward (If you have used Blender you'll understand that joke)
The barrel that I created was a perspective unwrap. Basically taking the current position of the model at that time and creating the wire outline. and I textured off of that. on the new model I am working on I am using the same process but I am seperating the pieces and doubling up the identicle parts. That's how I UVmap..
Just try a bunch of diferent methods and find what works for you.
Cheers,
RF