I agree with the guy above who says that BB rows don't hit the lats that well: I did BB rows for years before concluding that. As mentioned, it's too easy to cheat, which removes the positive effects. That's aside from the problems some people have involving the lower back-it's not necessarily a lack of strength but rather a structural problem; some just don't have the build to take that direct downwards pressure. As far as back angle, go with whatever's comfortable, there are no rules.
There are other free-weight alternatives such as T-bars & one-arm rows that from my experience work the lats more intensely, are easier on the lower back; because of the angle of T-bars, I don't find there's the same direct downwards pressure on the lower back, and there are also considerable advantages in grip variations. On one-arms you can take pressure off the back by bracing with the other arm, which also prevents cheating, and the grip can be varied by turning the wrist in any direction.
The only versions of BB rows that I've found helpful are more isolated-bracing your head against something to remove cheating (being careful not to injure the neck in the process), or BB rows done lying face down on a bench.
BB rows are considered by some one of the fundamentals by some, but I'm talking about effectiveness vs. following what's supposed to work.