Good subject. Almost everyone has the magnesium wheels (40-60), because when you order them, that is what you get. But if you order 40-60A's (or 199-71A for the complete conversion kit instead of the normal 199-71), these are aluminum, and corrode much less. The A's cost, and weigh a little more, but are worth it.
Also, disassemble and clean the wheel assemblies often. Especially with tundra tires which are not changed very often, and run in the water/mud, this gets overlooked. This is a great project to do on a cold winters day when the skis are on. Disassemble, clean and inspect everything, remove any corrosion, and use lots of zinc cromate. Pay a lot of attention to anywhere dissimilar metals come in contact with each other, (bearing races, through bolts, and brake disk. Replace through bolts if the cad plating is wore off. Paint each dissimilar metal surface with zinc cromate and allow to throughly dry before reassembling. Then coat with grease (LPS3, Corrosion X works too) like TJ said.