If you are trying to minimize taxes (vs. gaining access to funds as Miss. Mud. and So Close answered) then you want the conversions to occur when your taxable income will be lowest.
I will have to re-read through this conversion process, but I thought you put monies into the Traditional IRA and then convert to ROTH very soon thereafter to minimize any capital gain that would be taxed? Isn't it only the gain that is taxed when converting?
I believe you may be thinking of a backdoor Roth IRA. That's a strategy to put money in a Roth account when your income is too high to do so in the normal way. The way this works is that you put
non-deductible contributions into a traditional IRA and then roll it over to a Roth account. As you say, the sooner you do the Roth conversion, the less tax you have to pay on this money because it has not had time to grow much (if at all).
The Roth pipeline, on the other hand, mostly applies to
pre-tax money (i.e. contributions that you took a tax deduction for when you made them). The purpose of this is to get penalty-free access to these funds before age 59½. See, normally when you withdraw pre-tax money from an IRA or 401(k) before age 59½, you have to pay income tax on this money plus a 10% early withdrawal penalty. However you can convert this money to Roth at any time and only pay income tax (no penalty). Then if you wait at least five years before withdrawing the amount you converted, you can get that money out without paying the 10% penalty then either.
In a simplistic scenario where you're still working and you have all of your savings in pre-tax IRA/401(k) accounts, you would want to start the Roth pipeline five years before retirement. You would convert enough that year to cover your expected spending in your first year of retirement. Then four years before you retire you would convert enough to cover your expected spending in your second year of retirement, and so on until you convert enough for the fifth year of retirement during your last year on the job. After you retire you would continue this process until you turn 55, always converting enough to pay your bills five years in the future.
The problem with this is that you're doing Roth conversions for five years while you're still working -- this means you're likely paying tax at a pretty high rate on these conversions. Depending on your circumstances you may find that you would pay less to simply withdraw the money directly from your traditional IRA when you retire, paying the penalty and all.
Many of us are different from the "simple" scenario I laid out above. We may have taxable assets that we can spend for the first few years of retirement, which would reduce or eliminate the need to do Roth conversions while we're still working. We may already have contributed directly to a Roth account; these contributions can be withdrawn penalty-free at any time, which would also reduce the need to do Roth conversions while working.
In your case, if your husband leaves his job after turning 55 you can simply withdraw from the 401(k) without penalty at that time, so a Roth pipeline may not be a great idea for you. You may decide to do Roth conversions anyway to minimize future taxation on that money and reduce the likelihood of required minimum distributions, but the pipeline strategy as a way to gain early access to your money does not necessarily apply.