When it comes to the age-old question of ROTH vs Traditional, I always suggest some of the same ideas that pertain to investing: diversification.
Neither you nor the country knows what the future tax rates will be. As such, it makes sense to diversify your accounts to take advantage of whatever the future holds - unless you like taking extreme chances and don't mind having 90% of your retirement accounts in one over the other.
And remember - the diversification will help not only with specific tax rates, but tax policies themselves: remember that Social Security used to be just (at most) 50% taxable, because you already paid income taxes on your half while you were working. The 50% of the SS check you received that was taxed was your employer's contribution that hadn't been subject to income tax.
However, the gov't changed that a number of years ago, such that now (depending on your income) up to 85% of your SS income is taxable. In essence, SS recipients are being double-taxed on up to 35% of their SS income. However, those currently collecting SS have seen a real increase in the value of their SS contributions, compared to what they're receiving now...it's those of us who are 40 and under who will be really screwed, as our SS contributions will likely not keep up with inflation when we finally receive them down the road, AND we will be double taxed on up to 35% of it (unless they increase that again!).
My verbose point?
Just because ROTH IRAs/401(k)s are tax-free now, don't assume it's guaranteed that they would be forever tax-free. The gov't could very well institute an 'income tax', or even an 'asset tax' on ROTHs in the future. If they could do it to the sacred cow of Social Security, they can do it to anything.
my wife and I are currently in the 15% bracket so we're trying to "book" that tax rate now in a Roth 401k (and Roth IRAs) as we don't know what our tax rate will necessarily be in ER. Would you still advise against that?
If you're currently in the 15% tax bracket, don't overlook the SAVER'S CREDIT
http://www.irs.gov/publications/p590/ch05.html! Worth up to $1,000 single/$2,000 married in tax CREDITS (not deductions!), in addition to deducting your 401k/IRA contributions.