401K to Roth rollover and when to pay the taxes

[Timberwolf]

We find ourselves in a good position to take advantage of lower tax rates to do a 401k to Roth conversion.  My DH recently retired from a company where he has a modest sized 401k.  We will have pensions and social security to cover our living expenses amply so most likely won't need the 401k for living expenses.  We are looking at using the Roth for inheritance money for my DH's 4 grown children.  My question is, when do we pay the taxes on the conversion if we do (part of it) in December of the year.  We are in a very low tax bracket now (purposely, artificially) and won't have to pay income taxes until we start drawing social security in 5 years or so.

My only concern, really, is if we will generate a tax penalty for not having any withholdings.  Should we pay some estimated taxes or not?

Thanks!