I suspect that glass, typically being an amorphous structure, would be as isotropic as possible, depending on the heat-treatment. And it will be ideally elastic until it breaks, as there is no typical plastic deformation.
That said, there can be a lot of internal built in stress, depending again on the heat treatment of the glass. I think genuine Pyrex is heat treated to provide a significant compressive stress at the surface.
The other factor in typical glass work is that surface defects can, in the absence of tempering, reduce the strength of the item by orders of magnitude. As The New Science of Strong Materials author mentions, freshly drawn glass fiber has a tensile strength in the million psi range, but rapidly looses this due to microscopic scratches and settles to less than 10 ksi. It's a small book and entertaining/informative. http://press.princeton.edu/titles/8225.html
