The random event almost always comes up, so I usually leave the ruin until I get the event.
Sometimes the tech is useless, like Divine Right, but usually it's useful like Engineering.
I pave over the ruin if:
* I get the event (because I've never gotten it twice)
* it takes a lonnng time and I get tired of waiting
* the ruin is in a city with an important National Wonder like Oxford or Heroic Epic -- then I might really want to work the tile