Its not necessary. Some cog and chain and chainline and tension combinations to tend to make some noise back there. What you can do:

-ignore it.
-wait for the cog/chain to wear in together and get along better.
-maybe look at your chainline again.
-maybe consider whether the chain is too tight. yes, it can be too tight.
and, my favorite:
-lube the hell out of the chain for a while. It will quiet most noises, and eventually all will settle in together.

Greasing the cogs? The only downside is that sticky grease will just tend to muck up your chain. But maybe it will quiet things. I would just over-oil the chain, however.

jim