I turn my rig off, it boots up fast enough that that isn't an issue. Plus I don't want a glowing red behemoth with an admittedly loud pump running through the night in the room where I sleep :lol:.
Basically been said already. Make sure to pick quality parts (particularly on PSU and Case), clean and maintain the thing on a decent schedule (every 6months or so).
If power outages, brownouts or lightning strikes are common in your area, may want to invest in a UPS. That will stop the machine from powering down in case of an outage, giving you a good 5min or so to save whatever your doing and shut down properly. Will also protect against surges and any "dirty" power from the wall.
Also make sure to pick a good quality case that you like the look of, 5yrs isn't exactly a small amount of time and do you dont want to have an eyesore of a case.
That being said, some of the more mechanical parts like fans and HDD's will likely fail after that amount of time. So dont expect a perfect streak of things working over the rigs lifespan. One of my RAM sticks is only 6months old, just died out of nowhere.