Some LCD shield has dedicated 5v input for hooking up external 5v source. USB most likely won't be enough to handle 5" or bigger shield off Mega or Uno and generally voltage regulator won't work if the input is 5v, it'd need to be about 7v or more.
Some LCD shield has dedicated 5v input for hooking up external 5v source. USB most likely won't be enough to handle 5" or bigger shield off Mega or Uno and generally voltage regulator won't work if the input is 5v, it'd need to be about 7v or more.