New York (NY) Times sportswriter Arthur Daley wrote in September 1950:
 
“Perhaps there is something to the ancient but stern admonition: ‘No cheering in the press box, please.’”
 
The sports journalism adage became famous after the publication of Jerome Holtzman’s classic book of interviews with sportswriters, No Cheering in the Press Box (1974). People in the press box are supposed to be impartial journalists, not fans.  It’s not known where the saying (also given as “no rooting in the press box”) originated, but it has long been popular in baseball.
 
The March 2011 Huffington Post story, “No Cheering in the Press Box, Please,” did not involve sports at all and warned journalists not to root for political developments in their stories.
 
   
Wikipedia: Jerome Holzman
Jerome Holtzman (July 12, 1926 – July 19, 2008) was an American sportswriter known for his writings on baseball who served as the official historian for Major League Baseball from 1999 until his death.
(...)
Books
Holtzman wrote or edited more than a dozen books, including No Cheering in the Press Box, a collection of interviews with 18 sportswriters that was published in 1974. A revised edition in 1995 added interviews with six new subjects.
 
29 September 1950, New York (NY) Times, “Sports of The Times: Study in Slow Motion” by Arthur Daley, pg. 38:
Perhaps there is something to the ancient but stern admonition: “No cheering in the press box, please.”
 
Google News Archive
12 August 1955, The News-Dispatch (Jeannette, PA), “Today’s Sports Parade: Pitcher In Press Box” by Oscar Fraley, pg. 12, col. 4:
NEW YORK—Traditionally, there’s no rooting in the press box; but allowances are being made for Bob Grum, who “dies a million deaths with the Yankees every day.”
 
26 August 1969, The Times (San Mateo, CA), pg. 19, col. 2:
Speaking of Sports:
No Cheering In The Press Box
By JACK BLUTH
THE PRESS BOX AT Denver’s Mile High Stadium, scene of Saturday’s 49ers-Broncos thing, is unique. Sportswriters are asked (a) to please use the ashtrays and (b) refrain from cheering for the team of their choice. The 49ers’ press had no problem behaving itself. One had to admire the remarkable restraint demonstrated by the Denver writers.
 
29 September 1971, Daily Mail (Charleston, WV), “All Bases” by Bill Smith, pg. 35, col. 1:
No Cheering?
(...)
Loyal West Virginia writers perched in the press box are beside themselves. They cheer.
 
SUDDENLY, a voice comes on the press box loudspeaker, “There will be NO CHEERING in the press box.” It is the voice of Pitt sports publicity director.
 
The writers mumble, but they are quiet. The game continues.
 
OCLC WorldCat record
No cheering in the press box
Author: Jerome Holtzman
Publisher: New York : Holt, Rinehart and Winston, ©1974.
Edition/Format:  Book : Biography : English : 1st ed
 
Gambit (BestOfNewOrleans.com)
Bobby Hebert Has Supreme Press Box Etiquette
Posted by Alejandro de los Rios on Tue, Oct 6, 2009 at 8:09 AM
Of all the differences between New Orleans and the rest of the country, my favorite has to be the unabashed pride people in all walks of life have for this city and anything related to it. The Saints press box is no exception. Before every game, an announcement is made that reminds people of the old sports journalism adage: “There is no cheering in the press box.” Bobby Hebert somehow didn’t get that memo and people not from New Orleans seem tickled that something like this is allowed to happen. Meanwhile, people from Louisiana get a real kick out of it.
 
TVNewser
No Cheering in the Press Box and Other Words of Wisdom From Chris Jones
By Noah Davis on February 21, 2011 11:06 AM
Chris Jones, he of the brilliant Son of a Bold Venture, offers up some advice for a young reporter spending his first year in a pressbox.
 
SB Nation
No Cheering In The Press Box – Ever
by Jeff Gluck • Feb 23, 2011 9:39 AM EST
The cardinal rule of sports writing is simple: No cheering in the press box.
 
Along with No autographs, this is the unbreakable, non-negotiable standard by which all sports writers must abide. It doesn’t matter whether you’re writing for the New York Times, The Podunk County Weekly or you operate a web site out of your parents’ basement.

If you are credentialed as media at a sporting event, YOU DO NOT CHEER IN THE PRESS BOX. It’s very simple, really.
   
The Huffington Post
No Cheering in the Press Box, Please
By Terence Smith
Posted: 03/ 4/11 05:15 PM ET
Here we go again.
 
These days, the mainstream media are openly cheerleading for the rebel forces in Libya. Before that, they were in love with the demonstrators who occupied Pearl Square in Bahrain. And before that, the protesters who brought down the regime of Hosni Mubarak. And even before that, the crowds who sent Tunisia’s Zine el-Abedine Ben Ali packing.
 
Google Books
Exploring Journalism and the Media
By Lorrie Lynch
Mason, OH: South-Western; Andover: Cengage Learning [distributor]
2012
Pg. 257:
NO CHEERING IN THE PRESS BOX
Ask any sports writer about that motto “No cheering in the press box,” and you’ll probably also hear some variation such as, “If you want to have fun, go buy a ticket.”