Disconnect the driveshaft.
With the engine stopped the counter shaft will not be spinning. In fact none of the gears will be spinning, only the mainshaft and synchro hubs as the mainshaft passes through each of the gears and into the back of the input shaft. So that's three sets of needle bearings, one for each gear on the mainshaft, and the front mainshaft support that will be spinning at high speed differentials for extended periods that they were never designed for with potentially reduced lubrication.
I know people claim success in towing standard trans cars on the ground, but then again you never hear about the trans overhaul the car needed 10-15K miles later. Just like the guy who brags that his 1/2 ton pickup was just fine after being loaded down to the bump stops with sheetrock/concrete/river rock but fails to mention the rear axle rebuild needed a few months later after a wheel bearing started coming apart and metal contaminated every last item in the axle.