It seems to be dependant upon whether a keyboard is capable of generating and transmitting note velocity.
The detailed MIDI 1.0 specification says the following (I hope it's okay to quote the spec?):
MIDI 1.0 Detailed Specification 4.2 11, spanning pages 10 and 11.
"The three methods of using Note-On (9nH) or Note-Off (8nH) are as follows:
1. For a keyboard which does not implement Velocity, the note will be turned on using 9n,
kkkkkkk, 64 (40H) and may be turned off using 9n, 0kkkkkkk, 00000000 or 8n, 0kkkkkkk,
0xxxxxxx (a value of 64 [40H] is used for x).
2. For a keyboard which incorporates Key On Velocity, but not Release Velocity the note is
turned on using 9n 0kkkkkkk, 0vvvvvvv and may be turned off using 9n, 0kkkkkkk, 00000000 or
8n, 0kkkkkkk, 0xxxxxxx (a value of 64 (40H) is recommended for x).
3. Where the keyboard implements both Key On Velocity and Release Velocity, a note is turned
on using 9n 0kkkkkkk, 0vvvvvvv, and turned off using 8n, 0kkkkkkk, 0vvvvvvv."