Where in the dropouts?
 

On my 73 sports tourer with derailer hanger and horizontal dropouts, where should The rear wheel be in the dropouts? All the way back, forward, or in the middle?

I recently saw a Peugeot with a little two piece spacer and think that's a spiffy item to keep the wheel in the same spot forward in the dropout.

Usually keeping the hub centered over the derailleur hanger gives the best shifting performance, I believe.

Start near the center or per Joe Dog's recommendation. You can sometimes squeeze in a slightly larger rear cog by sliding the wheel back a bit, and you can very slightly sharpen the bike's turning circle and handling by sliding it forward, but I just tend to leave it approximately mid-slot most of the time. Also, those who want to put the widest possible tires on a close-coupled frame will probably want to try moving the wheel all the way back.