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Originally Posted by Synchrodyne
I was under the impression that a balun at the aerial itself also helped reduce noise pickup on the coax outer.
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Yes, if the antenna is balanced and the cable is unbalanced.
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On the other hand, I think it likely that FM aerials have used folded dipoles for many, many years, in which case the (readily available) 300R-to-75R balun might be more appropriate.
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Not necessarily. The main reason Yagis use a folded dipole is to raise the impedance, as a 'bare' Yagi often has a low feedpoint impedance. The folded dipole also increases the bandwidth a little, but this is mainly set by the lengths and spacings of the other elements. A folded dipole on its own will need a 300-75 balun; a folded dipole in a Yagi probably may need a 75-75 balun (if the maker has omitted it).