From all the reports I've read, and speaking to a few phone techs, I've roughly come to the conclusion that you should power down your phone once a week OR as needed. This falls under the same rational thinking as with your desktop computers.
The main reason to power them down daily would be to not waste energy. Along side that it allows your RAM to flush itself as well as "rest" when fully off, this gives the RAM some extra longevity. Diagnostics are often ran during the time of booting, so small errors are often automatically fixed. And of course the obvious reason; for updates requiring reboots.
Here's where things get interesting. Because the S5 has Ultra Power Saving, using it will practically negate the first argument of conserving power. Also you don't use nearly as much RAM resources in Ultra Power Saving mode which also pushes back the second point. So basically you're left with the last point; rebooting for updates
All that being said, you really only need to reboot when necessary OR you feel your phone needs a quick reboot. Aside from that, there's not much other reason unless you're clearing cache or simply want to give your phone a break. Again, the same reasoning can be expressed on desktop computers.
Again, this is assuming your phone isn't over-heating or anything of that nature.
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