I would prefer I-Bonds in taxable, and hold something else in your IRA.
The principles of tax-efficient fund placement are based on an assumption that you have equal options. However, I-Bonds are currently better than the bonds you can buy in an IRA, as they yield 0% above inflation, which is better than you can earn on short-term TIPS. In addition, they have the advantage of tax deferral, and they are immune to interest-rate risk; if I-Bond and TIPS rates rise, you can sell your I-Bonds at face value (with a small penalty if sold before five years) and buy new bonds, while if you had bought marketable bonds, they would lose value.
(Conversely, the equal options work in the other direction if you work for the US Government, as you can then use the G fund in the TSP as an emergency fund.)