[TenTec] About 3 prong plugs???

[OTAKEBI at aol.com]

I am changing out some 2 prong (non grounded) plugs with three  prong 
(grounded) plugs on some equipment to make the equipment more  safe.
I have been told that the black wire should go to the fuse side and the  
black to the switch.
Of course the green should go to cabinet.
But, my question is does it really make a difference as to which wire  (black 
or white) goes to the fuse side of the equipment.
I can't see how it would make a difference.
Any advice?
Dan/N4VET