Fuel wash is caused by an excessive amount of fuel in the fuel/air mixture.
The pumper carb is actually more efficient at metering fuel in the cycle, so there's less likelihood of fuel wash.
Of course, it's always possible to have too much of a good thing - so I guess if you increased the size of the nozzle, and the throw of the rod on the pump, then also increased the size of the mainjet and moved the needle to a higher position--you'd have problems. Of course, the bike would run like crud too!
Basically, fuel wash is easy to avoid - if the bike runs well and doesn't exhibit excessively rich symptoms - this is ONE thing you'll never have to worry about.
Even a bike jetted relatively rich (say 2 sizes too large on the mainjet) will not fuel wash - it'll just run like crud!