There is a lot of MATH that needs to be done BEFORE you do any conversions... and - frankly - it needs to be done with an accountant. Your tax bracket - all the limitations - etc need to be worked out.
If it works out -- you have to pay the taxes due OUT OF POCKET not from the 401 funds.... so it's an extra expense.
The big key to the whole thing is the NON TAXABLE WITHDRAWALS upon retirement. That can be huge... but all this needs some serious math and thought.
To me -- the key is just to save as much as you possibly can in the 401 AND contribute to a ROTH as well... and so what if you pay some minimal taxes once you're retired? That's the whole key is that you should be in a lower or minimal tax bracket once retired.