Location: Minneapolis/St. Paul, Minnesota - Latitude 45.057 Longitude -93.074
Rewards Points: 2,226
cinder bloc shed - fill or no fill ?
If you have a good foundation are in hurricane/tornado country, put a concrete roof on it and fill the cores to live longer (FEMA "Safe Cell"). If not, don't worry since it is still many, many times stronger than a stick built shed.
Why not 8, 9 or 10' high? My son did a 10' and has almost much more storage, especially for dead /seasonal items. - He was faced with a 120 sf maximum.
It'll certainly stay up if you don't, but I'd slug at least some of the voids full and incorporate some rebar as well. For what you're doing I don't think it is necessary to fill every void, but I'd do every third block horizontally, so you have some vertical rebars encapsulated in concrete top to bottom.