United, who play at Bramall Lane, are to allow Evans, 25, to train with them.
But United manager Nigel Clough said it had "nowhere near been decided" whether or not to re-sign Evans.
"Those in positions of influence should respect the role they play in young people's lives and set a good example," said Olympic champion Ennis-Hill.
"If Evans was to be re-signed by the club it would completely contradict these beliefs."
Evans was freed last month after serving half of a five-year jail sentence.
Three club patrons have now resigned while one of United's sponsors has said it will end its association with the club if they re-sign Evans.
Almost 157,000 people have signed a petition urging Sheffield United not to take the striker back.
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The club chose not to renew his contract following his conviction and he is currently out of contract.
Evans has scored 59 goals in 167 appearances during spells with Manchester City, Norwich City and Sheffield United.
He was sentenced to five years in April 2012 for raping a 19-year-old woman in a hotel room in May 2011, having been found guilty by a jury at Caernarfon Crown Court.
At his trial, he admitted having sex with the woman but denied rape.
He has declined any request to be interviewed by the BBC, but released a video statement on his personal website in which he said he wanted to play again.
Jessica Ennis-Hill wants her name to be removed from a stand named after her by Sheffield United if the club offers convicted rapist Ched Evans a contract.