I have several cinder block stands, but I won't put a tank on blocks unless I have an extremly rigid wood frame under the tank. With cinder blocks, esp. on a potentially uneven surface, there is a possibilty that all the legs won't be exactly the same height, providing uneven support if you put the tank directly on them. I don't like the idea of the stresses placed on the tanks if one leg is even 1/8" shorter or taller than the others.
So I build a 2x4 frame that is well-braced and extremly rigid. I use a couple strips of foam weatherstripping (the firm kind) between the tank and the frame. After the tank is set on the frame, and the frame on the cinder blocks, I place shims between the cinder blocks and the frame until the tank/frame unit is level. (I do this when the tank is empty, then check again as the tank fills to make sure the tank doesn't settle unevenly as weight is applied.)
More than anything, I want to avoid any racking or twisting of the tank, so a solid frame is pretty important to me.
HTH,
Jim