I agree with those who describe the use of this expression as being more in the symbolic category than anything else; and that it is used to represent the generally unskilled or semi-skilled occupations that JWs are found to be employed in. In other words, as with the elders, so with the rank and file -courtesy of the WTS's anti education stance.
Of all the elders that I ever knew, none were window washers by occupation. There were a number who did general cleaning (i.e. janitorial) work, including, but not limited to, windows. A few held carpet cleaning franchises, and the JWs in general did seem to be a little over-represented in pest control work. Also, there was a trend in the 1970s and 80s for JWs to be employed as school caretakers (which, to be sure, would have included a fair share of window cleaning!). Other than that, there were retired farmers (with more money than the bull could $h#t, but to be fair, I never heard any of them give a talk about materialism!); there were also carpenters, plasterers, truck drivers, factory hands, labourers, timber workers, electricity meter readers, orchard hands, boilermakers, plus a few who were best described as "Jacks of all Trades, but Masters of None".
Very, very few of the JWs I ever knew had university degrees - one Ministerial Servant was a doctor, another an electrical engineer (his father, an elder, somehow remained one, despite sending his son to university), and a third was a civil engineer (whom the building committee was quick to grab hold of whenever there was a Kingdom Hall or Assembly Hall project coming up).
None of the elders I ever knew had a university degree, and as others here have pointed out already,were singularly unqualified to deal with many of the people problems that they encounter - such as my daughter's attempted suicide back in 1998. (By the time the office cleaner, boilermaker and labourer who comprised the JC had finished with her, the miracle is my daughter did not then go out and finish the job!)
Bill