The extra SMB provides the neutralization capacity for subsequent oxygen uptake. The art of the matter is determining how much extra SMB you need in your system to provide adequate protection. Sure, 100 ppm SMB is probably a safe starting point, but it comes with an increased cost in terms of Na and SO4 content. Some brewers on the German forum have been experimenting with lower dosage, including me.
In my opinion, modest Na content in beer does not produce a really detrimental taste effect. If you can keep that below 50 ppm, it should be OK for virtually all styles. You shouldn't taste it as salty, but it could have a sweetening effect.