The Bogleheads' Guide to Investing

it’s possible that some investors who buy I Bonds with very low guaranteed real return rates and then cash them in when they’re in a high tax bracket could possibly end up with less spending power than they started with. However, since I Bonds are tax-deferred for up to 30 years, many investors should be able to postpone cashing in their I Bonds until after they’re retired and are in a lower tax bracket.