It makes no difference to the internals of the transmission.

The only difference in wear is to the shift linkage.

Putting it in neutral or park also stops you or at least slows you down from taking evasive action in the event of a pending collision, so it is considered a bad practice in some jurisdictions.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue