I align them to the grid for ease of use - the tiles I supply in my signature should be easy to cut out and put together ... In the real world, things don't nescesarily fit to a 5 foot grid, but in Dungeons and Dragons we pretend the world is like that - its so much easier

oh... and Ravells has a very good point - something that makes random (or not so random) cities in vector would be very cool indeed.