How will reducing and freezing affect the flavor of stock?
I've certainly never detected any adverse effects on the flavour from freezing and/or reducing stock. 

However I no longer reduce stock before freezing as it seems like a waste to boil off loads of water only to add it later, when the only benefit is saving surprisingly little freezer space. It comes out a little more concentrated from my new slow cooker anyway, as it's hotter than the old one.

Stock is simmered for hours anyway; getting it very slightly hotter for a few more hours won't do any harm.