Huh?  Did I miss something?  We're talking about a
power supply here,
 right? 
  A power supply should provide the specified voltage.
 The power supply must be rated at *at least* the current (Amps) or
 power (Watts) drawn by the load, but an excess here does not hurt. 
This is true only of well-regulated supplies.
Wall-warts (which is the form of power supply I thought we were talking
about here) often are not well-regulated.  A wall-wart rated at 6V
750mA is reasonably likely to supply well over 6V if used to power a
device that draws only 150mA.
Sometimes this doesn't matter.  But sometimes it does.
Of course, if we're not talking about wall-warts, but instead talking
about properly regulated supplyes, never mind.
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