I am utterly perplexed as to the logic behind the suggestion. The advantage of converting is that the income the converted (Roth) IRA generates will never be taxable. Converting a small sum now should lead to noticeably larger, and untaxed, sums in the future (assuming a long future). Converting on a year-by-year basis in retirement would be as close to having no benefit whatsoever as I could possibly imagine, as you have to pay taxes on the amount converted in the year it is converted -- instead, just withdraw (from the traditional IRA) and spend it, after paying the taxes.
Do some of the math on that one. Tax the amount needed to withdraw to have 25k income per year for 60 years. Then tax the lump sum necessary to provide 25k income per year for 60 years. You will probably pay less taxes on converting. I don't have time to outline the math atm.
I agree that you shouldn't convert 1.5 million dollars into a Roth all in one year, and on a smaller scale potentially spread out conversions of smaller sums to a Roth with tax considerations in mind. But I think the point that Bogart and I are trying to make is that the main tax advantage of a Roth is the potential for tax-free growth over a long period of time. If you wait until a far-off retirement, convert just a small amount and then use it soon after you convert, you didn't really gain much from doing that and you might as well just spend it.
For example, I'll assume you have $10,000 in your traditional IRA right now. You retire in 22 years and let's say it's now $30,000 in today's dollars (assuming 5% interest beyond inflation). Assuming tax brackets remain the same, $4500 (15 percent of $30,000) is almost double 25 percent of $10,000. The difference becomes more marked the longer you wait, thanks to the beauty of compound interest. Then if you convert in retirement and end up using it 5 years later, all you gained from the conversion was not having to pay taxes on the interest for those 5 years, and you lost the flexibility to use it during those 5 years while you waited for it to season. So you do need to consider how far off retirement really is to figure out how many extra years of taxable interest growth you would have and how many years you would have post-conversion until you needed to use it. It may mean you don't end up converting at all and just withdrawing from it as you need it in retirement when you're in a lower tax bracket. But if you are going to convert, it may make more sense to do it sooner than later to get more of the benefits of long-term tax-free interest growth.
Also keep in mind that after the 5 years of seasoning, a Roth IRA offers a little more flexibility with early withdrawals and doesn't have the forced distributions of a traditional IRA. I think you first have to answer *why* you want to convert your IRA into a Roth - to meet which financial goals - and then decide on your timing based on that.
Sorry this got long-winded, but I'm not sure if I got my point across with my earlier post.