With the HE-400? Absolutely not. I can see no advantage of having the vent on the HE-400 unless you want thumping bass (EDM or rap). I do however recommend it on the HE-500.
EDIT: I think I need to further clarify the fact that the vent mod does not have the same effect on the HE-400 and the HE-500.
Oh, I was talking about the HE-500 so I guess it is worth it. Would anyone happen to have a link to a good foam that I could use for the backvent mod as I can't seem to find any.