You don't HAVE to alter the natural shape of the stave unless you cannot lay it out so that the string will bisect the handle. The stave can wiggle back and forth along its length, rise and fall, even a little twist is ok... in fact, many bowyers hold such character endearing in their bows.
For the beginner, it may be of benefit, due to lack of understanding/experience, for the tips to be equally positioned in relation to the handle. In other words, if one tip is even with the handle but the other has 2" of string follow, it would likely benefit him to heat it, and make it even with the handle as well.... not necessary though.