Echoing Seekers comment, ask them to not cut the steerer - Yet !.

They can install spacers above the stem as required, unless it's stupid long, then they can do an initial cut, install extra spacers and let you ride for a while. That gives you the options to raise or lower the stem until a good position is found.

Then you can have it cut to that position.

If they want to charge you each and every time (the first cut should be included with the build) you can always buy a Park Tool Headset Cutting Guide, they run about $25 or so and allow a parallel cut with a hacksaw.