The way I read it, technically speaking no placards are required at all as long as the info is recorded somewhere in the aircraft. The one exception is the passenger warning placard that's required IAW AC 20-27G, para 10.c. Here’s the abbreviated FAR:
§ 91.9 Civil aircraft flight manual, marking, and placard requirements.
(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
(b) No person may operate a U.S.-registered civil aircraft—
(2) For which an Airplane or Rotorcraft Flight Manual is not required by § 21.5 of this chapter, unless there is available in the aircraft a current approved Airplane or Rotorcraft Flight Manual, approved manual material, markings, and placards, or any combination thereof.
Furthermore, here’s what Order 8130.2G has to say:
4104.b.(13) This aircraft must contain the placards or markings, as required by 14 CFR § 91.9. In addition, the placards and markings must be inspected for legibility and clarity, and the associated systems inspected for easy access and operation, to ensure they function as intended by the amateur builder/owner during each condition inspection.
So, the only thing I can find that is an absolute must (besides the pax warning mentioned above) is the word “EXPERIMENTAL” has to be displayed IAW 8130.2G sub para 4104.b.14.
However, much like the issue that crops up time to time with a mag compass, it might be easier to get through the certification inspection to at least temporarily add the placards and take’m off later if you really don’t want to display them.
