Time to climb on my soap box.
I've seen several posters comment that they run alloy wheels (presumably aluminum; magnesium are pretty rare and really pricey) in the winter with no side effects. You may not see much damage in the first few years, but if you want to see what happens, just keep doing it. You'll see.
I have a 1995 Ford Escort with the stock Ford aluminum alloy rims, not exactly top quality but functional. I run all-season tires year-round on that car using the stock rims. After 9 winters the rims are more-or-less destroyed.
You can summarize the effects as follows:
First 4 winters: no obvious side effects.
After winter #5: started to observe pitting along the edge of the bead and some bubbling of the shiny coating. I probably would have developed air leaks but I replaced the tires at this stage so the rims were all buffed out.
After winter #6: More of the same. No air leaks yet.
After winter #7: Pitting along bead is very obvious; material starts to flake off edges of tire rim. Start getting slow air leaks in two tires (1-3 psi per week).
After winter #8: Damage to rim continues. Air leaks increase (5-7 psi per week) and occur in all tires. Start repairing leaks.
After winter #9: Rims look awful. Shiny finish material is flaking off all over. Air leaks start occurring again, as much as 5-6 psi per day. Leak repairs performed several times over the last 12 months as tires develop leaks.
It doesn't matter whether I have them repaired at Crappy Tire or a quality tire shop, the problems just go away for awhile and eventually come back.
To date, I've spent, by best estimate, over $300 re-installing, rim-buffing, and otherwise recovering from damage to the rims, and that only solves the functional problems. Cosmetically, the rims are a mess. Some rims are worse than others. One rim/tire combo has been "repaired" four times in two years for air leaks. One rim has only been done once and it's still holding, unless you count the fact that it took them three tries in three days to get it to stop leaking in the first place.
If you use one set of rims for both summer and winter duty with dedicated summer and winter tires, you will at least benefit from the repeated reinstalling, assuming of course the rim survives the repeated remove/install cycles. However, you can expect to see cosmetic damage starting to show after 4-5 years and rim-buffing will probably be needed from then on.
All this depends, of course, on how much salt your wheels are subjected to. The chemical warfare geniuses who do the roads here in Ottawa prefer the salt approach rather than plowing it much of the time.
Chemically speaking, the damage is done at "warmer" (above freezing) temperatures. Below freezing, the salt is non-reactive and may coat things but won't damage them. The killer is those thaw times when the temperature is near freezing and there's plenty of winter salt around. (I read once that the worst damage is done as the temperature approaches 10 degrees celsius, but I cannot confirm the accuracy of that statement.)
I use winter steel/summer alloy on my newer vehicles (Windstar and Mazda 3). Having never seen salt, the rims on my Windstar still look fine, but they're just getting to the age now where wear would show so I'll have a better idea going forward. The 3 is of course way too new to judge, but I have my 16" steels with Toyo Garits for winter duty there.
Overall, I'd have to say I'm a convert to winter tires on dedicated, steel rims. The steel may be heavier than the alloys and the handling may not be as confident, but you shouldn't be pushing things that hard in the winter months. Save it for warm weather when you can put the good hardware on and the weather is conducive to such shenanigans.
Your mileage may vary. I post this as evidence from my own circumstances. Read or ridicule at your discretion.
