I'm curious as to why hard wiring the buttons will be less precise?
Is the USB trigger doing something different than triggering with the button?
Unlike old mechanical cameras, current digital cameras use software to scan the switches on the camera - including the shutter button. When you short the shutter switch on each camera, that action has to be picked up by the camera software and processed. The time this takes varies quite a bit depending on what else the camera microprocessor is doing at that point in time and on what further processing is needed. And that variation can be pronounced when you try to trigger multiple cameras, causing very poor sync as each camera takes its own path through the software prior to actually releasing the shutter. You only have to google for "shutter lag" to understand how bad this can be.
With precision sync, CHDK allows each camera to start its shot but then "halts" the photo shooting process until just before the actual shutter is released & the shot taken. It waits for the USB 5V signal to go away and then triggers the shot. So when you use the USB trigger, there is a minimum of lag between the 5V going to 0V and the actual shot and thus precision sync