If you're going to do long runs of DC, just make sure you have high enough gauge wire. Voltage drop is a lot more significant for DC than it is for AC. That's why your transformer is usually right before the first light. For example, if your lights are 12v and require 5A of power, at 50 feet you loose 3v with 18 gauge wire but only 0.8v with 12 gauge. So, don't cheap out on the good stuff when it comes to cabling for them.
There is a big advantage to using only low-voltage outside though. The wire can be direct buried and isn't required to be run inside a hardened conduit (at least in the US).
