measure twice … etc. (or assemble the thing uncut and mark the headtube where you think it’ll need cutting)
Do a common sense check as well. Measure your frame’s head tube, add the depth of your stem and add a bit for luck. Make bloody sure that your cutting mark isn’t a long way off that.
(also longer steerers make forks more sellable when you’re done with them)
I have a couple of uncut steerers on bikes, just because I haven’t got round to it. Tiny bit more dangerous If I fall on it but CBA to sort anytime soon (neither uncut steerer is on a bike I’m likely to have a massive crash on)
I do the 2 stems thing too – one is a ded old one that cracked the faceplate and stripped a thread, the other is whatever I’m using on the new fork