Here's an interesting article I just found on ground planes and how currents flow etc.
I've not been too careful about where I run traces and it hasn't really bitten me in any noticeable way--yet--but I will be paying more attention to this particularly for analog and high frequency, neither of which I do much of; but that'll soon change I suspect.
Another thing to keep in mind (and search for articles on) are bypass capacitors and their placement and effects on noise.
As to the 45 degree bends, I am kind of OCD about that. I've seen some debates on that. Just keep in mind there are multiple viewpoints. Here's a contrarian one.
You can find more articles on the topic, both viewpoints.
Hope this helps.