The answer in milliseconds is: MOX NIX!
IMHO, based on 50+ years of struggle with similar issues, whether the
wing washin is plus inboard or minus outboard is not important.
The AIRPLANE will seek trim equilibrium no matter which method is applied.
I have seen and had gliders with each setup work equally well as to thermalling ability.
Conversely, several of my F1A compatriats and I have tried flat
mains with ONLY tip differential and found them generally to
be dogs in thermal condition.
Therein lies one reason to have picked the right glider for any given flight.
DIFFERENTIAL is the key, i.e., having just the amount of positive incidence on the inboard wing to do the job you desire.
As John L stated on his answer to your post: 'As aspect ratios increased, there is less wash needed to produce the same
result, because of increased moments.'
Hence, I have found that a 2 meter glider can and must use more differential than a 2.5 meter glider.
Examples from my fleet: 2.1m Wishbone has about 0.4 deg washin, my 2.25m Li'l ALs have 0.2-.3 deg, my newest,
2.4m EOS has ~0.1 deg, while my 2.5m Buntero is about the same.
All the above is with flat main panels that have been trimmed using
wing wigglers, not panel twists.
Mind you, the ears have washout. Usually the left[I glide to right]ear
has a bit more than the right, or inboard ear.
This goes back to my comment above related to the job the glider is intended for.
Is it a thermal pig, where a bit more drag won't matter, or is it your ultimate FO bird, where any added drag
may mean enuf more sink to be an all-so-ran?
FLY, MAX, WIN!