Sorry for the long post, but this is how I understand it.
The reason for selecting the correct gear for the direction facing when parked. on some engines that have a hydraulic cam belt/chain tensioner. During normal operation, the tensioner is on the undriven or slack side of the belt/chain system. if the engine is used as the holding force, it will turn as the compression bleeds off. If the crank turns backwards to its normal direction, due to the incorrect gear being selected. this now pulls on the slack side and without oil pressure to hold the tensioner firm the tensioner collapses. Now the next time the engine is started, it has to re-tension the chain with the chance of skipping a tooth worst case, or just noisy operation for a few seconds at best. The NSX does not have this issue because the belt tension is fixed and should not vary. So match the gear to the direction the car will roll; if pointing down hill, first gear. If pointing up hill, reverse.