300 stainless is not a higher grade, it's just a different grade. Nickle is added mostly to retain the austinetic structure and make the steel more malleable. although it does also add a little to the corrosion resistance. Because it is malleable it is used to form sinks, dishwasher doors, and other parts requiring severe forming operations. A sink made out of 410 (a martinsitic heat treatable magnetic stainless)would be significantly more expensive to manufacture than a 302 stainless sink.

If your refrigerator door is magnetic, then one of two things occured:
1. The manufacturer went to a lot of extra expense to form the panel out of a 400 grade of stainless to make a panel that was stronger and more scratch resistant. (not likely).

2. The panel was formed out of a laminated material, thin layer of 300 stainless over heavier layer of plain low carbon steel. This also could be laminated after forming, although that seems unlikely.

The chances of any item that requires severe forming like a sink or dishwasher door with wrap around sides, being made out of anything but 300 series stainless is just about nil. of the common grades, 316 generaly (but not always) gives the best corrosion resistance.

Major manufacturers use such a large volume of sheet stainless steel that they can and probably do dictate there own specs and composition requirements to the steel mills. The alloy may not conform to any of the standard grades. I'm sure they use an austinetic stainless material, but it may not be 302, 304, etc..