There's something I've always wondered about unwrapping UVs. Are you supposed to always keep the UVs arranged neatly into rectangular sections or is it okay to relax them?
as long as the texture is in the bounds of the UV and there is enough texture space it doesn't matter.
You square them out to maximize memory usage, but relaxing them causes less warping and looks better. Depends on your needs.
Depends. You usually try to "keep the shape" - for organic models, your UVs would look somewhat similar to a skin rug, for machine parts your UVs would be somewhat rectangular.
For cars, your UVs of the sides would just look like someone took a photo directly from the side, without stretching the silhouette - very useful for paintjobs.
Keep in mind that painting patterns or "geometry-aligned" shapes will be much easier if you straighten your UVs. Also, baked stuff will look better because there won't be as much aliasing (or blurriness due to antialiasing).
Threw together a few layouts to illustrate what I'm talking about,
I always straighten and grid-snap everything if the topology allows
more info here