Tom:

What I've always done is cut the bottom at an angle that allows for
the tack strip at the bottom.

This angle is similar to what I do at the top, which allows me to
stand up a 120" box in a 120" space.

It keeps the rest of the work going in the right direction.

How to best measure this angle? Do you level off the back
of the tack strip to where the bookcase will end in the
front and then just figure it out?

These and the other suggestions are quite helpful. I think
I will go with the leveling blocks in front. I'm not
after a build-in look for the cases, so it doesn't
matter if the wrap around the base molding. I came to
that conclusion just recently. On other cases I will
want a built-in look, so I will cut the molding to
fit the case. But for those, it will be on hardwood
floors, so carpet is involved.

Thanks for all of the replies.

MJ Wallace