Safety and Law are what we are talking about.
Regardless of law, one needs to decide if they are willing to compromise safety for an aftermarket wheel. Since all airbags are "supplimentary" and I have been in several very serious accidents without an airbag, you come to realize how effective your seatbelts really are. A 5pt harness definitely helps. This decision is personal and I've decided long ago to remove my airbag.
Laws... well it's law that car mfg equip all cars with "supplimentry" restraint systems after 1990. The law, as Stu indicates, does not require that you drive with an SRS. Vehicle certification requires SRS to be verified. The only time a vehicle is required to be certified is if it changes ownership. So... if you plan to sell the car, keep your SRS parts. Until the law changes such that it is UNLAWFULL to have a non-functional SRS, it's not a concern. I can't see such a law being passed unless people start dying by the truck loads due to non-functioning SRS or removed SRS while still using thier seatbelts. This is unlikely.
Insurance has nothing to do with the law. Each insurance company has thier own policies around claims. If you crash with your seatbelt on and smash your face against your aftermarket wheel, you probably won't be able to make any health claims. If your seat belt fails, more damage will be done by the SRS than the wheel...