Tax-exempt munis probably do not belong in most people's portfolios.
First, if one has any equities in tax-advantaged accounts, they can be sold to make room for more fixed income investments. That is, all your tax-advantaged accounts would need to be completely filled with fixed income so that you have no more room for fixed income in tax-sheltered accounts and needed to put fixed income in taxable accounts.
Second, one needs to look at the after-tax return of tax-exempt fixed income investments versus the after-tax return of regular fixed income investments. You want to get the higher return after taxes are paid. Since tax-exempt munis pay lower interest rates than taxable bonds, one would generally need to be in a high marginal income tax bracket above 28% to make this worthwhile. But do the calculation for your situation. In the 25% tax bracket, you generally make/keep more money by using regular fixed income and paying the taxes.
Third, note that tax-exempt interest is included in your income when determining how much of your social security benefits are taxed.