I have always assumed that the 24 pin had grounds on it. I have made cases before out of cardboard boxes and one out of a 5 gallon bucket. I didn't put any additional grounds on them and they ran solid for a year or till I sold them.

eLiTe_sLaYeR
I'm thinking about building a desk.  As long as I have the MB on the brass standoffs, I should be good right?  Does the bottom of the desk actually have to be metal with metal legs for it to be technically grounded?  I'm kind of dumb when it comes to electricity.  Thanks for your time.  

Hi eLiTe,
 
 Both of the posts above are correct. And I will confirm,,,The answer to your questions are yes, you are good and no, it does not have to be metal. The motherboard is powered by DC (direct current) low voltage, and does not require grounding to earth. That is done through the power supply. But the stand offs are a good idea because they will protect the components on the back of the motherboard. The material you mount them to is irrelevant. The feet of the case do not require grounding to earth either, most computers have rubber feet. The power supply has its own ground as stated in the post above and does not require being mounted to a metal or grounded surface.
 
Please come back and post some pictures in the appropriate forum when you are finished. We all like to see stuff like that! And theres a Mods Rigs site here to show it off!
 
Good luck with the build!
 
Any question about electricity is a good one!

Yes, the motherboard should be placed on a metal tray with metal standoffs and metals screws connecting the motherboard to those metal standoffs. The metal motherboard tray should also have a wire connecting the metal motherboard tray to the power supply's metal case.

It will probably work just fine without doing all that, but to do it properly, it should all be grounded. Without the motherboard grounded, you may have a lot more issues with static electricity when you plug in USB devices or use your keyboard and mouse. A burst of static electricity to an ungrounded motherboard can cause the system to lock up randomly or even damage/corrupt/erase your motherboard's BIOS. Flash EEPROM is very sensitive to static shock. Computer cases are made of metal for a reason and everything in a standard computer case is grounded for a reason. Sure, the motherboard is grounded through the PSU wiring to the motherboard, but that isn't a very good ground because it tends to float at a higher potential than ground when your system is under load. Without the motherboard grounded to a metal motherboard tray which is grounded directly to the PSU's metal case, the distance between devices on the motherboard relative to the distance they are from the 24-pin connector varies and the devices further away will be at different voltage potential than devices closer to the 24-pin connector. Just sloppy and not preferable overall...