It doesn't matter when you withdraw the money, the taxes will be the same. $10k earning 5% compounded monthly will be $16,470 in 10 years. After paying 1/3 of the gains in taxes, you'll be left with $14,313 which has a present value of $10,446 at 3.2% inflation.

So yeah opportunity costs must be accounted for, but in your scenario the opportunity cost is negligible. We're talking 0.4% per year. Investing in a CD is like pissing in the ocean