Unless you are matching a current bike and you know exactly how high you want the bars, cut the steerer at least 20 mm too long and put spacers above the extra steerer length to do the headset adjustment.

Ride it that way for a few time and decide if the bars are too low, too high or just right. Adjust as needed (without cutting the steerer) and ride a few more times. After, and only after, you are certain about the bar height, then make the final steerer cut.

As DaveSSS recommended make the steerer cut about 2 mm above the top of the stem and add a 5 mm spacer above the stem to get the needed headset adjustment gap. That assures the stem is fully supported by the steerer tube.