If you have space between the motherboard tray and the back cover, run as many there as you can. Depending on the brand of PSU you have they all should be long enough To come back inside the case right where they are needed. I use Corsair PSU's and the only cable I have to use an extension on ate the ATX 8 pin cables. You can buy one for a couple of dollars. The small slots in the board are for cable ties, which have already been mentioned. I can't imagine trying to do cable management without them.I use extensions, but just for looks with the sleeved cables. It is not necessary. But here is a build with decent cable management.