Hacking and stabbing shield's replicas suggest that it's indeed the case.
http://www.youtube.com/watch?v=HeBPDVfi_DI
Given the way thin planks of wood tend to behave, it's not surprising at all.
If somebody wanted to give a good hit to the shield, he certainly had to be vary of possibility that his edge will get stopped.
As far as metal shields go, main problem is big expense of metal, so weren't generally considered worthwhile trough most of the shields history, until state of metallurgical industry made them more accessible.
They will also tend to end heavy for the size, steel is dense, and must still have proper thickness and bulk to not bend to easily.