I am also given the impression that the asteroids in a given Belt location are not random, and will yield the same resources each visit should you return to the same rock. Obviously it's easier to locate the same rock in the nodes that are depleted or low reserves and hence have very few rocks. Thus far I've not bothered systematic cataloging myself, aside from noting "X System Inner Belt Node 2 at least two Pt roids", thus picking that node to return to later. Still takes a little hunting again to find the Pt rocks, but my spatial memory is fairly decent.