There are other Roth conversion threads on this aspect, especially ones that reference the Santa Clara professor who wrote an article suggesting they were a maybe at best as well. It is situation-dependent. For example, with a couple of kids in college and the ability to claim the AOTC, I can do Roth conversions in the low five figure range at 0% federal tax. Since I'll be probably pay 26% at age 75 between federal and IRMAA (which the SSA has me at about a 70% chance of reaching), it's more than a maybe for me personally.
Interesting. I always thought the tax torpedo was the high marginal rates in one's 70s and beyond due to large RMDs and SS, for those with significant traditional IRAs and SS benefit amounts. For me I do Roth conversions (and maybe QCD's; we'll see) to try to stay out of the 32% (plus about 13% IRMAA for a total of about 45%) bracket in case I make it to my 80's. I thought Ed Slott coined the term and used it as I describe in some of his books.