The first time I saw it done was not long after I got my printer & some guy was printing working prosthetic hands for kids. The section that attached to the arm was printed flat then warmed up and bent round the recipients arm for a perfect fit.
Sometimes it is much easier to print flat or level then bend later, I also used this method when printing a new nose cone for the Aure BA1 glider, the fuselage was a rather odd elliptical shape & it was the only way I could think of to get the correct fit after printing it round.