If you have just bought the ASUS DC Top card and are running it as ASUS shipped it from the factory, you're fine.
Not all chips are created equal, some are able to run high frequencies without excessive power draw, while other chips from the same batch might run hotter and/or require higher voltages to achieve the same frequency. Basically when a AMD or Nvidia release GPUs they decide on a "stock" frequency based on the competition they're facing and how many chips they're prepared to throw away or re-purpose. Setting a higher 'stock' frequency will give the cards better performance, but fewer chips are going to make grade.
Companies like ASUS then come along and buy a whole bunch of chips. They test them and put the best ones in premium products (like your DC top), with the lesser chips going into lower tier parts.
Personally, I trust ASUS to not provide you with a GPU that's running so close to its limit that it's likely to die prematurely. If you're serious about making your GPU last as long as possible, I'd do some googling on lowering the voltage, as I understand it, it tends to be high voltages that limit a product's life. Still, I think you're worrying too much. Starting to mess with frequencies and voltages when you don't fully understand what you're doing is a recipe for trouble. I'd trust the Asus engineers!