No. The valve covers, engine dress up kit, fuel door and push start button wont affect the car's performance or reliability in any way. Just make it better to look at.
The only things that void the warranty of the car is an aftermarket part in place of an OEM part. Example, if you buy a a cold air intake, then the warranty for your cold air intake is obviously voided. Nothing else is voided, but if that cold air intake causes, say, the engine to somehow blow up and they give you proof that the cold air intake caused the engine to fail, then it voids the engine too. But other than that, the only thing that is voided is the aftermarket part. I'm actually quite curious that you thought dress up parts would void your warranty in that way, but no big deal. Have fun dressing up the car!
That good ol' red (Boss 302), white (GT/CS) and blue (5.0)!
2008 Mustang GT/CS: 5 speed | FRPP 4.10 Gears | Bama 93H Tune | Magnaflow Magnapacks | Bassani O/R X Pipe | Airaid CAI | MGW | J&M LCAs | Saleens 19x8.5