“Be nice to the janitor. You may be working for him tomorrow” (business adage)

There are many different ways of the business saying: “Be kind to those working under you; you might be working under them some day.”
 
“Do not antagonize the newsboy today—you may be working for him tomorrow” was printed in the Sunday Bee (Omaha, NE) on August 7, 1910. “Don’t assume just because a man’s working for you that he’s your personal slave; in five years you might be working for him” was printed in the Cordova (AK) Daily Times on June 2, 1921. “Never ignore the office boy. You may be working for him some day” was printed in the Pittsburgh (PA) Press on August 17, 1932. “Be nice to the office boy. One day you might be working for him…” was printed in the Columbus (GA) Enquirer on April 11, 1966.
 
“Newsboy” or “office boy” is sometimes replaced with “janitor.” “Be kind to the janitor, for tomorrow he may be your boss” was printed in The Capital Times (Madison, WI) on November 30, 1959. “Always be kind to the janitor, you never know when you’ll be working for him” was printed in the Boston (MA) Globe on February 20, 2018.
 
Hollywood (the entertainment industry) has its versions of the saying. “The Hollywood adage, ‘Never swear at a prop boy, you may be working for him tomorrow’” was printed in the The Daily Missoulian (Missoula, MT) on March 25, 1931. “Old Hollywood Proverb: ‘Always be nice to the guy working for you because tomorrow he may be your boss’” was printed in The Daily Home News (New Brunswick, NJ) on November 27, 1943.
 
     
Newspapers.com
7 August 1910, Sunday Bee (Omaha, NE), pg. 4 banner headline:
Remember the Newsboy is a Potential Magnate and Scorn Him Not
(Cols. 6-7—ed.)
Tim Murphy, the actor, once remarked “Do not antagonize the newsboy today—you may be working for him tomorrow.”
     
2 June 1921, Cordova (AK) Daily Times, “Pep Paragraphs,” pg. 4, col.2:
Don’t assume just because a man’s working for you that he’s your personal slave; in five years you might be working for him.
 
25 March 1931, The Daily Missoulian (Missoula, MT), “Screen Life In Hollywood” by Hubbard Keavy, pg. 4, col. 4:
Hollywood.—Proof that the Hollywood adage, “Never swear at a prop boy, you may be working for him tomorrow,” is a truism as seen in the case of Rowland Brown.
 
17 August 1932, Pittsburgh (PA) Press, “Seckatary Hawkins—Daddy Mulholland’s Weekly Message,” pg. 25, col. 1:
Never ignore the office boy. You may be working for him some day.
 
30 January 1936, Crane (MO) Chronicle, pg. 7, col. 3:
Don’t be too raw with the man who is working for you today. You may be working for him, tomorrow.
 
27 November 1943, The Daily Home News (New Brunswick, NJ), “Hollywood Highlights” by Erskine Johnson, pg. 4, col. 6:
Old Hollywood Proverb: “Always be nice to the guy working for you because tomorrow he may be your boss.”
 
Google Books
6 November 1948, The Billboard (Cincinnati, OH), “Ballyhoo Bros. Circulating Expo” by Starr De Belle, pg. 75, col. 3:
Third Stool, Ark.
October 30, 1948
Dear Editor:
That old saw, “Never laugh at a First of May because you may be working for him tomorrow,” continues to hold good.
   
30 November 1959, The Capital Times (Madison, WI), “Voice of the People: Derides the Plaint of The FCC” by R.B.S., pg. 34, col. 7:
In a business where it behooves one to “be kind to the janitor, for tomorrow he may be your boss,” there are few secrets.
 
11 April 1966, Columbus (GA) Enquirer, “Cairo Coach Raps Scoring” by Don Osborn, pg. 17, col. 1:
It reminds one of the old business adage: “Be nice to the office boy. One day you might be working for him…”
     
20 February 2018, Boston (MA) Globe, “MELVILLE, David Joseph—Chairman of WinterWyman Companies,” pg. B5, col. 3:
He spent his free time as a janitor at B.C.—ultimately becoming living proof of one of his favorite sayings: “Always be kind to the janitor, you never know when you’ll be working for him.”
   
Twitter
Bodeman44
@bodeman44
Replying to @thehonorablecsc
I was always told be nice to the janitor, he might be your boss someday.
10:21 AM · Oct 5, 2018·Twitter for Android
 
Twitter
Cherie Anne Bryant
@CherieAnneBrya2
Replying to @ScottJKyle1
Be nice to the janitor, he may be your boss someday.
11:22 AM · Dec 30, 2018·Twitter for iPhone