There is no hard and fast rule in the NEC but there are recommendations. (Informational notes)
It is seen as a design issue, not a safety one.
Good electricians do try to have sound design practices tho. In the end, it might mean a lower bid accepted by a less than discerning customer. A lot will really depend on what is available at the service point. I am getting 124/248 so I can take a little more voltage drop than a customer on a utility that is providing voltage on the low end of the accepted nominal. I still try to stay within the recommendation.

210.19(A)
Quote
Informational Note No. 4: Conductors for branch circuits as
defined in Article 100, sized to prevent a voltage drop exceeding
3 percent at the farthest outlet of power, heating, and lighting
loads, or combinations of such loads, and where the maximum
total voltage drop on both feeders and branch circuits to the
farthest outlet does not exceed 5 percent, provide reasonable
efficiency of operation.


215.2(A)
Quote
Informational Note No. 2: Conductors for feeders, as defined in
Article 100, sized to prevent a voltage drop exceeding 3 percent
at the farthest outlet of power, heating, and lighting loads, or
combinations of such loads, and where the maximum total voltage
drop on both feeders and branch circuits to the farthest
outlet does not exceed 5 percent, will provide reasonable efficiency
of operation.


Greg Fretwell