I don't know what your source is, but I highly doubt it's true. I have a aluminum mountain bike and I have quite a bit of steel (not stainless steel) parts on it. I don't have any problem with corrision. Mind you, the crank and chainring I've got are aluminum and the chain itself is steel.
The only real thing you need to look out for is titanium nuts and bolts (and screws) on titanium parts.
