Which bonds to cash?

I have to cash some savings bonds for the upcoming college semester. What is the best way to do this? Do I use the rate or the yield at the time I want to cash them? Tom's response In general, cash the ones with the lowest rate, which compares how your bonds will do in the future. The yield is the bond's lifetime yield, which tells you which ones did best for you in the past, but often the ones with strong past results will have weak future results because of the way rates are calculated on older bonds. If you have EE bonds that were issued since May 1997, try to get as many as possible to the five-year mark before you cash them. For I bonds, cash the ones with the lowest fixed-base rates.