Here is an example of why it may not make a difference?  It all depends on your tax rate when you retire.

For example say your taxable income  is 170,000 for 2017.

For 2018, if everything remains the same you will be in a 24% tax bracket (2018 24% bracket is 165,001 to 315,000).

So  you convert 100,000 to a ROTH from your SEP.  If the tax is paid out of the converted funds (24% +6% VA), now there is 70,000 starting in the ROTH.  Notice that if you converted instead 200,000 to a ROTH,  this would put you in a 32% bracket.

So the choice is leaving the 100,000 in the SEP and growing to be taxed at a later date,

OR

paying the tax now and having 70,000 to invest tax free.  Assume the same rate of return of 6% for both options.   Assume you and spouse are 79 and will live another 12 years per the joint annuity IRS table.

70,000   in 12 year  is worth 140,854

Leaving in SEP the 100,000 is worth 201,220.  Pay the tax at 30% =  60,366. So after taxes the amount is 140,854 (201,220 – 60,366)..  If you or you beneficiary are still in a 30% (IRS and VA) bracket, then no difference.  If you or your beneficiary are in a lower bracket then leaving the money in the SEP is best.

The \$64 dollar questions is what will be you  tax bracket when your retire or for your beneficiaries when you die ( for 2018 the rates are  22% 77,401 – 165,000  and the 12% bracket is 19,051 – 77,400).

Why people recommend converting is they assuming that the cash to pay the immediate tax bill will come from another source of funds and the ROTH account will start out with the 100,000.