My day job is as an engineer specializing in corrosion on the USAF T-6 Texan II trainer so I thought i would put in my two cents. This plane has Alodine, chromate primer inside and out and a two-part topcoat. as well as wet installed fasteners and sealed faying surfaces and we still have corrosion issues in some locations.
Corpus Christi is the worst we get all kinds of corrosion there but even other location in non-severe locations have corrosion wherever the coatings are damaged it just takes longer to appear. we have some aircraft there less than 15 years old that are having to get major repairs (new wings on some).
Granted our aircraft are used pretty rough and stored outside under covered shelters instead of hangers but my experience so far has been that you need some kind of corrosion protection. I would make sure to pay special attention to coating any areas that will be difficult to inspect on a regular basis and you should make sure that you do a thorough corrosion inspection as part of your annual maintenance. especially in areas where steel and aluminum parts touch. We have also started using Cor-Ban 35 quite a bit and Cor-ban 27L on latches and stuff (27L doesn't dry)
I am considering building an RV-9A currently and I will be priming all of my parts. Even though I live in Oklahoma and that is a low severity area I am planning enough trips to Florida the Bahamas etc. that I do not want to end up with major corrosion issues 10 or 15 years down the road. Of course, I am only 35 so my plane is going to have to last me a while