It depends on whether you expect to sell the frame in the future. If so, you don't want to cut the steerer tube, because it will affect resale price. Otherwise, typically what people do is to cut it down gradually in increments, always leaving some spacers above the stem to permit future changes in stem / bar position. I've got all my bikes set up with between 10mm and 15mm of spacers above the stem. However, I never have more than 20mm of spacers below the stem, and if I require more height to the bars, I'll get a stem with more rise and flip it up.