Asked by TV's "Entertainment Tonight" about Armstrong's confession last week that he used banned substances and blood transfusions to win seven Tour de France titles, the Grammy Award-winning singer said, it has "got to be really hard to walk around knowing that you're not telling the truth about something."
"I think that honesty is always the best bet and that the truth will set you free," she said. "To carry a weight like that around would be devastating in the long run... I always contend that the truth is the best way to go."
The celebrity couple dated from 2003-06.
Crow did not say she was aware of Armstrong's unsportsmanlike and illegal strategies for winning at the time they were together.
Armstrong long denied he cheated in races, even after he was stripped of his Tour de France titles and banned for life from competing by the governing body of cycling in 2012. He finally came clean and apologized for his behavior in an interview with Oprah Winfrey last week.