I have just finished 12 blocks and i’m ready to put them together.
The question is ‘to sash or not to sash’. Is there a rule?

Answer:

Definitely personal choice but you might also be governed by how big you want the quilt to be;  and are you adding more borders?   In which the 1st outside sashing could be considered to be a border – see The First Sampler Quilt series if you want to know what I mean.

Also it depends on the block:  by putting them together do you get an interesting secondary pattern?   or are there more seams that need matching that you don’t want?!?!?