Are you trying to optimize your Roth conversions across your lifetime or just have an estimate of this year's taxes? The tax software is going to be more accurate for current year taxes, but using it over and over every year would be a lot of work and your situation changes as IRMAA, SS, RMDs all become part of your life. Also many elements in the tax code are adjusted for inflation, the TCJA tax cuts are set to expire and so forth, so I wouldn't use tax software for lifetime Roth planning.
If you are trying to optimize across your lifetime, then my suggestions from simplest to most comprehensive are:
I-orp extended - It can give you a reasonable feel for a Roth conversion plan without a lot of work. It is missing some accuracy in taxes but is easy to use and results will usually be in the ballpark. A missing piece is the two year look back for calculating IRMAA, so for instance i-orp uses income at age 65 instead of age 63 to calculate medicare costs at age 65.
Retiree Portfolio Model - spreadsheet available at bogleheads.org. Very detailed input required and it's not always obvious where you input things. Covers more of the tax code than i-orp but still missing a few details and requires you to input the Roth conversion amounts each year, it does not try to make a plan. You can set your heir's estimated tax rate when they withdraw anything that's left so with some work you can look at different strategies on an equal footing since a $ left in tax deferred is less valuable than a $ in other accounts. Interesting if you are nimble in spreadsheets.
For my own situation, I decided spending a few $ would be worth it, so I bought Pralana Gold ($99 1st year, I think $49 updates, requires MS Excel-no substitutes). Its tax module is much stronger and covers the things I was wanting like existing Long Term Capital Gains, capital loss carryover, non-deductible IRA contributions, inherited IRA, deduction phaseouts, AMT.
For Roth planning, Pralana Gold has a Roth optimizer. While I'm not totally in love with that feature, you can also mostly chart your own path by providing a % of the initial tax deferred account to convert, the tax brackets and IRMAA tiers to respect and letting it find the optimum path within your constraints. To estimate the tax impact of the remaining tax deferred balance for your heirs, it uses your average lifetime marginal tax rate.
It's a very detailed program like RPM, but things are well labeled and there is a good manual, so if you are familiar with the concepts, you can get the hang of it. I bought it 4-5 months ago and really like it, now I do all my planning with it.
After a lot of work, I came to the conclusion that in my case, there was a broad, flat optimum, as long as I converted at least 40% of my initial tax deferred account balance, the results were very close. But there are so many critical variables - account balances, rates of return, other income, life spans, etc. that no one should generalize.
