Reverse gear is not synchronized in most cars, except for really, really high dollar ones. This is what causes some drivers to grind briefly while shifting into reverse from neutral. If you always shift from neutral, into any forward gear and then into reverse, you will prevent this grinding. The forward gears are synchronized, so by shifting into a forward gear first, you are synchronizing the input shaft of the transmission to the speed of the car, which should always be zero before shifting to reverse.
Most cars have a lockout of some sort that prevents you from accidentally shifting into reverse (such as directly from 5th to reverse, if R is to the right and back). It is still very possible to attempt to shift into reverse (such as by going from 5th to neutral, then reverse) when you are going at a high rate of speed, but this will result in a horrible high frequency grinding noise, and unless you apply a lot of pressure, you are unlikely to succeed. If you do succeed that means that you just used an unsynchronized gear to bring the input shaft from 2000-3000 RPM forward to 8000-12,000 RPM backwards, and that?s not going to happen without consequences.