Depends what you mean by break-in - but iron blocks take longer to heat up than aluminum, so it takes a little longer to throw non-wrinkled bullets. I do what JumpingFrog told me, and that was to buy a single electric hot-plate to rest the blocks on and pre-heat them while the lead is melting down - saves a lot of time.
Rust... iron molds are typically blued, so they don't need a lot of care - just store them away from any humid environments and wipe the outer surfaces with a few drops of oil on a rag once in a while.
good info, thanks.
They did take longer to heat up, but the results look good.
I had to grind the Lee handles a bit before they'd fit the RCBS mould, but they work fine.