On E90Post, there's no real consensus as to whether rustproofing is needed because they doubled the corrosion warranty compared to the E46. I assume they upgraded the body panel materials to a more rust-resistant metal. Also note that the E90 has plastic under panels, but I'm not sure if that's a benefit or drawback (i.e. does it trap salt-laden moisture?). You see lots of rusty E36/E46's around, but not too many E90's. Is it just a matter of time? The E90 has been on the road for ~7 years now and I assume rust should be hitting cars within 4-5 years of winter driving (my experience).
There is still the added benefit of rust-proofing making dissassembly of nuts/bolts much easier: I never had any snap on my E36 which was Krown'ed annually. This benefit has to be weighed against having a cosmetic hassle of having constant drip from the doors onto the rockers that persists even into the summer.
Last edited by kenmar; 10-06-2013 at 10:51 AM.