Those of you without tilted monitors can test this by tilting the gun
No, it will not decrease the accuracy of the gun. It will make it slightly harder to aim, and I've never seen a gun game that uses a tilted monitor - but if you don't have a huge tilt, or if you are firing at an angle as well, I can't image it would have a huge effect.
Glass and lex, assuming they are not tinted or smoked, will only have a minimal effect. Choose carefully though, you want something that doesn't stop or bend light.