Generally speaking, 5% really isn’t enough to get my attention.
Excessive VD is just another “indicator.” In the majority of cases, it will simply indicate loose or poor connections.
I often use it (in my reporting) to show some of the effects of AL branch wiring when present.
The long and short of it - excessive VD is an indication of high resistance within the circuit. There are many possibilities as to the cause, but at a minimum, it reduces the efficiency of the circuit.
I know the NEC is pretty much mute with regard to VD in residential construction, but when I get readings of 10% or more on a 15A load, I get a bit “suspicious.” I’ve seen as high as 50% in new construction, which (IMHO) is a reportable defect.