If you're going to wear your show boots for schooling and practice, invest in the $15 or so boot galoshes, the little rubber slippers you put on over the foot of the boot. Put them on as soon as you dismount. I used to keep them on my boots at shows and not pull them off until I mounted. IME, most of the wear is on the shoe portion of the boot from walking around in them.
Wearing out the calves of tall boots is more likely if they're unlined or 3/4 lined; you have to balance the desire for the sleek look with the longevity. Kitten val, if you've purchased dressge style tall boots, they are usually fully lined and a little stiffer than a hunter style boot. Also, riding in a dressage seat, you don't get the constant wear of the stirrup leather against the boot like you do riding in two point, so you should not have to worry about wearing holes in the calves.