"In general, you must report the interest in income in the taxable year in which you redeemed the bonds to the extent you did not include the interest in income in a prior taxable year" This may help.
The tax is owed in the year of maturity regardless of whether or not you cash the bond. If you cash the bond in a year after the final maturity I believe the absolutely correct thing is to file an amended return.
I recall an IRS term called "constructive receipt." IOW even if you don't choose to receive taxable income, it is still taxable as of the date you could have chosen to receive it.