I have just worked out a way to unwrap models in Blender that produces a result that makes it much easier when creating and working with textures outside of Blender. I thought I'd better write it down before I forget.
The default options on my setup produce some strangely stretched and distorted results and it is difficult to fit them on the rectangular shaped textures. I have tried all the clever methods on the drop down list of unwrap choices and always end up using the standard unwrap even though I had not been entirely happy with the result.
Now, using the same menu selection, I have at last noticed the tools options panel appear at the bottom of the tool window, usually on the right hand side of a 3D window. If you have expanded the little plus and if your screen and Blender window are big enough to see the bottom of the main tool list!
"I think this is why Blender has a reputation for having a difficult UI."
The Unwrap window ONLY appears AFTER you have pressed the Unwrap button and goes away again as soon as you move on to another task!
Look out for this small set of options:
The bit that now makes the Unwrap do exactly as I would like, is the method, 'Conformal'
Don't ask me what that means I have no idea. What it does is to make all the shapes stay as close as possible in proportion to each other and not distorted. So a square remains a square and not a squashed rhomboid shape that I used to get. You have to change the default, 'Angle' option to 'Conformal', every time. I don't know how to default to 'Conformal' yet.
I find this arrangement much easier to work on in GIMP to align the textures to look correct. I use the Export UV menu item and use the result as a layer in GIMP and just hide it before saving the final texture.
Back to Blender...
As mentioned above, the odd thing about some Blender options is that they don't become visible until after you have performed the action. When you change the option, in the temporarily visible tool window, the result of the action is changed. In this case you Unwrap the model to a mess and then change the option and it all neatly lines up for you as you watch.
It can only do as good a job as the seams you have marked, so I spend a lot of time in advance ensuring that all sections of the model can be unwrapped to a flat shape without having to stretch too many of the edges..
It's a bit labourious but just select an edge or two or three and press the 'Mark Seam' button. Change your mind, select the same edge and press, you guessed it, 'Clear Seam'. You can Unwrap again and again, to try it out.
Anyway, as I started out by saying, I have found a UV Unwrap method I am happy with.