A board should use traces as needed by the design. The bottom preference is mainly "historical" as it was needed for added mechanical strength for through hole parts. While you CAN solder non-SMD devices to the top side (aka component side) of a board, you lose a good amount of structural support (IE; think about mounting non SMD capacitors/resistors and IC's using only TOPSIDE copper). You do gain the bonus of not having to drill any holes if you do that. I imagine it would be easy to break a part off the board... pulling the copper... (Anyone for the lazy solution?)
On a DUAL sided board, it's pretty much up to the designer to do what makes the most sense. If you grab some spare boards from around the house you will likely find that boards have a directional preference. IE; one side will be used primarily for North/South vias while the other side will be used for East/West vias. To me, this approach makes the most sense.