Answer:
For loan rates, Reg Z does not specify a number of decimal places, but if you round to one or even zero decimal places, you risk putting your stated APR outside of tolerance to the actual APR. For example, if your actual APR is 5.4444 and you round to just 5% APR, you'd be outside of tolerance on a regular transaction.
For deposit rates, Reg DD requires APY be rounded to 2 decimal places. The corresponding interest rate can be expressed to more than 2 decimal places. (12 CFR 1030.3(f)).
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