The Durablocks do need to be trued, at least mine did.
I put some 80 grit sticky paper on a flat surface and sanded the
bottoms of mine, worked really good.
I also have a wood block that I true up every once in a while
the same way.
That wood block gives the truest straightest sanding of all
my blocks. So simple, yet so good. (for dry sanding)
Sometimes the simple stuff is the best.
A wood block can be any size and shape you want.
You just have to check it once in a while for trueness,
but that's an easy fix.
I don't know why I don't use it more often.