My suggestion - use a tempo/mood indication (such as Allegro, Andante etc) instead of a mm marking. They're much more useful anyway.
Sometimes more precision is desired.
Tempo is a function of heartbeat (so it's perception will depend on what's preceded). Some pianos sound faster than others. Some rooms sound faster than others.Don't confuse precision with accuracy.1 + 1 = 2.0000003143637 is still wrong.