I was working on a map commission awhile back for someone who wanted an Earth-analog with the terrain modeled as realistically as possible. (That he wanted me to render it in what's probably my most artistic style - the one I use that's easily the least suited for that sort of thing - should have tipped me off right off that bat that he didn't know what it was he really wanted.)
Anyway, he sent me this big list of requirements, and I buried my head in the books and maps for a few days to get it right. When I sent him the first draft of the map, he came back with this rambling list of complaints, including one that, based on what he knew about geography, a certain river in this worlds version of Asia couldn't possibly be going the way I had it.
I pointed out to him that that particular river was, in fact, the Yangtze, which really does flow just like that.
Shortly after that I stopped working on that particular map.
Long story short, I'd just echo what Alaskanflyboy and Nomadic said. The list of simple rules that Nomadic posted is a perfectly good set of rules for 99% of the gaming world out there and will get you more than close enough for horseshoes and hand grenades.
(One good thing about working on that map though. I learned a lot about India's rivers; fascinating stuff, really.)