I've always wondered why commentators always see a trade of dragon and top tower to be more advantageous towards the dragon. Tower is worth about the same as dragon, but losing a tower is also a loss of vision and map control, so isn't top tower worth more?
Because killing a tower doesn't prevent the other side from getting the same pot of gold, whereas killing dragon does. A ward at dragon is all well and good but the only way to be sure the other team doesn't get it is by killing it.
It's the same reason m5 have done so well in Kiev and Hannover, they counterjungle relentlessly because it's gold you can earn that also prevents the other team from getting it. Basically, in the big race to be the richest team, gold from neutral sources counts double!
The way I see it, the top and even bot towers are not too important as they will usually always go down at some point in the game. Also I'm pretty sure dragon gives more gold than a turret. I would say as long as it's not a mid turret dragon is a good choice.
Counter Logic Gaming (CLG) doesn't contest dragon and would rather farm and have lane advantage and they are one of the top teams. However, most other top teams contest the dragon so it could go either way. Sorry I don't have an exact answer but honestly, it comes down to what you think is more important.
Actually dragon gives 190 gold instead of a turrets 150. SOURCE:
On top of that dragon is up every 6 minutes from it was killed. So it has more potential to generate more gold and exp for the team.
Dragon is worth 200g while a tower is only 150g.
Top and bottom lanes towers will eventually always go down every game and they aren't nearly as important as mid towers. Games are rarely won by having your opponents top or bottom towers down since those take much longer to get to than mid.
The biggest impact of taking the first tower in any lane is making it more dangerous for the enemy to farm the lane, due to less safety.
Whenever you ace the opponent you want to try and get as many of the mid lane towers down as you can.