Don't do it.
In order for it to make sense, you would have to assume you would be in a higher tax bracket when you retire than you are now. Also, you would have to do it incrementally to save on taxes. If you convert it all at once, the amount of your portfolio would be added to your current annual salary, which could push you into a higher tax bracket (depending on the size of your portfolio).
If you are considering a Roth IRA, open a separate Roth IRA (assuming you qualify with your current income) and max out your contributions each year. That way, at least part of your retirement portfolio will be tax-free.
My only concern would be your current tax bracket. With a Roth IRA, you will be paying taxes on the income at your current level (28%) - do you think you will be making more or less when you decide to retire? If you feel you will be at the same tax bracket or higher, I would recommend opening a Roth IRA for future contributions, but NOT converting the existing portfolio. If you feel you will be in a lower tax bracket, definitely stick with a traditional IRA.