My foundation is off just a bit but by the third row of blocks, it is a little too close for comfort on my level.

For dry stacking cinder blocks, is there any issue with just putting some morter under the part of the cinder that is off or should I do a mortar bed under the entire first row? My two sides are fine but the back row has a slight low spot.

I even thought about using shims on the back wall with the assumption that once I pour the concrete into the cores, it will solidify.

I am very new to this type of thing so thank you in advance for the help!