[Pharo-project] Should blocks passed into Metacello methods like #for:do: return spec?

Concerning Metacello's block-accepting methods, should the blocks passed in always return spec? I ask because some sends in the image pass in blocks where the last message sent is yourself to "spec", making that the return value of the block, while other blocks just fall off at the end without explicitly returning spec. Is there a reason why one should be preferred over the other?