Do you have locos stalling or hesitating between booster sections?
A booster common will definitely help this for sure. Think of it as a drain for any stray voltage or voltage differential between boosters.
Connect all the cases together along with the command station. DO NOT connect them to a cold water Earth ground! This is floating ground.
Next, you may have a DCC out of phase situation where the rails are not the same polarity from booster to booster.
This is easy and quick to check and resolve. Just swap the two track output wires right on the front of the booster.
Then run a loco across the gap. If the transition is seamless then your phasing is correct.
Try to make sure that your track voltage is the SAME across all boosters as close as possible. The actual number does not matter as long as it is the SAME number everywhere. I shoot for 14v for HO scale and 12v for N-scale. It should read the same right on the booster output terminals and everywhere on the track even under load.
You can pick any available screw on the exterior of the cases. I use the ones on the bottom that hold the cover on.
The PB5 does NOT have a voltage adjustment. The output is fixed at 13.8v using the switch on the power supply.
The PB105 AND the Ph-Pro command station both have an adjustable output from the rear of the box, you might need to take the cover off and use the pot inside if the hole does not line up 100% correct.
Best practice is to lower the PB105 and the PH-Pro from 14v or whatever it is set to DOWN to match the 13.8v output of the PB5.
The actual number does not matter, getting them as close is possible is the goal.