If you use the keystone adjustment you are no longer projecting a 1:1 pixel map of the incoming signal, i.e. you are reassigning what pixel goes where by a process called "scaling" which can often have subtle visual artifacts, so it is not ideal.
Panasonic isn't very explicit about it however several people besides just myself have pointed out that if at all possible it is best to project from no higher than the top edge of the screen [or bottom edge if projecting upward from a desk mounted position] to ensure the best image, especially when using the auto zoom feature for a constant image height when switching between 2.35 and 1.78 (16x9) aspect ratio material. Tilting the projector is not advised because the resulting projected image will be a trapezoid, not a rectangle, however in some instances you have to fudge things and tilting the projector and/or screen sometimes is your only way to correct for the less than optimal location you are attempting to project from.
In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".
Last edited by m. zillch; 09-08-2017 at 08:38 AM.