Tempering is generally not recommended for interior table tops (it is required by code for outdoor tables).  The tempering process may warp and distort the smoothness of the glass surface.  If tempered glass does break it will shatter into tiny pieces and vacate the space it occupied (not a pretty site at Thanksgiving dinner).  While tempered glass is stronger than un-tempered glass, the tempered glass is actually easier to break if impacted on its edge.  Also, normal warpage from tempering could cause a top to sit unevenly on its base.  Similarly, we do not recommend tempering for shelving because of its vulnerability to breakage along its edge and its tendency to vacate the opening, dropping its contents on the shelf below, possibly breaking it too.