The 5Vsb is used for a number of things, of which I believe all are optional (optional for you depending how you want to use it, PSU's in gerenal are required to have it to give the user the option)
Wake-On-Lan
Wake-On-Ring
Wake-On-PS2 (eg. power-button on your keyboard)
Wake-On-USB (again, power-on from USB keyboard, keep your optical mouse LED on while comp is off)
If you don't want any of these things, then you don't need to provide 5Vsb at all IMHO. Also, look in your MB documentation and you will likely find a few jumpers for things like whether USB should use 5V or 5Vsb. Depending on the amount of current you are designing this to allow over 5Vsb you may want to change some of those jumpers.
If this was a real server MB, the management processor would also run off 5Vsb, and the current required would likely be a lot higher.
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