Negative Wires for Parallel LED Strips
Since you have separate power supplies for each section, the ground (negative) wire will normally carry essentially the same current as the power supply, where essentially means +/- a negligible current due to the control circuit.

BUT, ground is a tricky beast, and there is the potential for a ground loop if things get weird. Imagine that your power supplies are not isolated, and the ground is connected to either neutral or earth through the AC input. Now, if there is a problem with the ground wire connecting to supply A, the current sourced by A will return through its neighbor B, into the AC line, and back out the AC into supply A. This means the ground to B would end up carrying the current supplied by B and A.

What I described is an extreme situation, but it is more likely that a few ohms of resistance would re-route a bit of the current through a lower resistance path using a different ground.

Short answer, 50A is a lot of current. For 5A supplies, I would be safe and have 10A capacity for everything (more important for the ground), and put 7A fuses in the ground path.