I don't depend on clamping squares for squaring per se, but I do use them a lot to keep pieces in place and more or less square while I'm assembling. They are very useful third hands. That said, I use the Rockler phenolic squares which I don't trust that much anyway, although they are very nice for the use to which I put them.
I'm seriously considering a set of the Woodpecker squares. They come in packs of 2, 4, and 8. I'm thinking 4. Can anyone think of a good reason to order 8?