Some argue that you do not know whether your taxes will be higher or lower in retirement. In order to best prepare for this, tax diversification is advocated by keeping a portion of your retirement in tax-free accounts and a portion in tax-deferred accounts. This is not bad advice, but one thing to remember is that traditional IRA and all 401(k) accounts have required minimum distributions (RMD) starting in the year you reach age 70 1/2. Depending on the amount of your RMD, it might push you into the next tax bracket. Having more of your retirement in a Roth IRA can help avoid this scenario. You can also consider rolling over a Roth 401(k) to a Roth IRA to avoid additional RMD amounts.
If your employer does not offer a Roth 401(k), request that they add the option. Roth accounts are a great example of delayed gratification as you are giving up a little cash now for the peace of mind of not having to worry about whether your taxes will be higher in retirement. If you can afford to give up the cash now, you probably are not going to regret it when you reach retirement and pay no taxes on your Roth distributions.
Additional reading: The Retirement Savings Move Tax Pros Love from the Bloomberg Personal Finance section.