Doing this is just one step you should take to ensure nothing goes wrong. It could be speaker wire, croc clips or even a bulldog clip, practically anything metal that is touching/attached to both the metal casing of your 360 and PC at the time they are connected by SATA.
The Xbox 360 uses whats called a floating point ground, but your PC will use a “true earth†ground. This difference COULD cause excess voltage to travel through your SATA cable and damage your Xbox 360 DVD drive and/or PC.
However, it is VERY unlikely that this will happen. I have flashed several consoles fine not taking this step, but to be extra careful you should really do this.
After all it will only take a minute! I'm just lazy! lol
ps: If you havent already done so, check out the downloads section and download and read Textbooks tutorial. It explains everything in there, including all the things to watch out for. I remember the first time I ever flashed a 360 DVD drive, I was crapping myself, but I studied the tutorial over and over, watched the videos of how it was done until I knew what I had to do & it was fine.
When you get it done yourself & you play your first backup, you'll be grinning from ear to ear.
Happy flashing! 8)