Most G5RV builders recommend at least 50' and prefer 75' which they say provides a more manageable SWR at the Radio end. ...
Remember that the G5RV was developed in the days of tube rigs with tunable output stages.
While this could match a reasonable range of impedances, it required a lot of capacitance
to match low impedances on 80m and 160m, which was often the limiting factor in the
matching range. For example, I remember one of the Johnson rigs was rated to match
from 40 to 600 ohms: this means it wouldn't match a 2 : 1 SWR at 25 ohms, but had no
problems with a 200 ohm load (SWR = 4 : 1).
A typical G5RV will show an impedance of 15 to 40 ohms at resonance in the 80m band at
the bottom of the matching section. Even at 25 ohms ( SWR = 2 : 1 ) this could be
difficult to match for some tube rigs. Adding a quarter wave of coax (about 50', depending
on the velocity factor) acts as a transformer to raise the impedance that the rig
sees (to 100 ohms in this case) and made it easier to match.
That's one reason why G5RV originally recommended the minimum coax length - they
weren't as worried about SWR in those days, just whether they could adjust the rig
to load into it.
But modern tuners are designed for a different purpose than a pi-network output network
on a tube transmitter. Most tuners will be reasonably efficient at 20 ohms or so (though
they can get lossy at very low impedances.) As long as your tuner can match the load
efficiently, using a short length of coax is a good idea as it lowers coax losses. However,
because losses are typically low on 80m anyway, it doesn't really make much difference:
the difference between 5' and 50' with a 20 ohm load is about 0.2dB.