I first started applying the parking before every shift to park when a Porsche 928 owner's manual told me to do so. My recollection is that it was quite emphatic about this - that failing to do so would result in harm to the transmission. My current MB 1992 owner's manual also advises to do so, in one case with a "warning": "firmly depress parking brake pedal", then "move selector lever to "p". It further says "Important": "It is advisable to set the parking brake whenever parking or leaving the vehicle", and "When parking on hills, always apply the parking brake". I find it easier to do it all the time, very easy habit to get into. Regular use seems to be good for a parking brake, they can get rusty and lock up from non-use.