Safe to use a common return for multiple DC power supplies?
It depends on the total current and what the cable is rated for

Let's say each 12 V LED strip consumes 12 W, then each LED strip has 1 A flowing through it. For each LED strip, the ground wire needs to carry 1 A, from left to right in your schematic it will show 1 A, 2 A, 3 A and 4 A at the bottom. 

Given my 12 W example you need a cable that is rated for 4 A, since you have 4 LED strips and 1 A per strip. According to powerstream you need 15 AWG or you can have several cables in parallel with higher AWG values.