Generally, I try not to use single letter variable names in any scope that doesn't fit into a single screen in my editor. That way, I don't need to remember what "f" means, it's right there in front of me. And I shouldn't have too many variables in that scope, so conflicts aren't all that important; if I really do, I just start using multi-letter names again.
This is a good heuristic. For me, single letter variables are almost always iterators in places where space is at a premium (i.e. one line list comprehensions in python). If I'm breaking a loop out over multiple lines (which is almost always the case), I prefer a longer, more informative variable name.
