I love that Dandarc and I just posted the same damn link 4 minutes apart.
Ok, on the taxable income front here's where I think the conversion ladder wins out in my case, and likely most cases:
When I FIRE (according to current plan at least) I will be 45, wife is 44, kid #1 is 13, kid #2 is 9. At that point they are both eligible for the child tax credit. They're both dependents. That's about it.
5 years later kid #1 will likely head off to college so I will have a big new tax deduction/credit. 4 years after that he will no longer be a dependent while #2 heads off to college. 4 years later neither is a dependent. This happens to be 13 years into a retirement that started at 45, so I'm 58.
I have a lot of capacity during the first 13 years to convert a ton from T.IRA to R.IRA filling up a big chunk of the 15% bracket and paying very low taxes. That opportunity ebbs and flows during those 13 years, it does not stay constant at 3.5% of my stache.
Beyond that 13 years I'm left with another 13 years to clear out my T.IRA to avoid RMD's. I plan to get very creative with all of this, and the SEPP does not allow me the flexibility I will desire to get this done in the most tax efficient manner.
I hope that further explains what I meant by rigid. If not, fire away with questions.