Expiration date is mostly for store use, so they know when to remove items from shelf by.
That stops people from claiming they just bought a 25 year old can of spam.
The 'Rule of thumb', and store labeling policy may have changed without me noticing, so I'm in for the discussion setting me straight.
ETA: All of the above except for tomato based stuffs.
They can tend to get a little metallic from the acidic nature of the product after several years.