"The savings in machining permitted by starting with 3/4 offsets the cost in my calculation. "
There is no less machining by using 3/4, you still have to make 2 cuts 1 for top bars and 1 for bottom bars. My 2nd cut gives me both a top bar and a bottom bar. I can buy 5 2x4x8 for about the same money as 1 1x12x8, so I can get 100 top and bottom bars to 50 top bars out of the 1x12.
"Leaving a last bit on bottom keeps the block from tipping during ripping to 3/8 width."
I started using a scrap of 3/16 under the relief to keep it from tipping.