I just bought a new Soma Rush frame (only). It's a 53cm frame (110mm headtube). The headset I am putting in has a 38mm stack height. That gives me a 148mm steerer tube length need. Soma's fork comes in a 140mm steerer tube length or 170mm. Is cutting down a 170mm a good option? Or should I try and space it out the 22 extra mm (0.866 inches)?

Just get the longer one and cut it down.

Hello I agree I would get the longer one and cut it, but if it was mine or a good customers frame I might stack/space the headset about 8-10 cm. I do this 1 beacause at some point a taller headset might get used. 2 since you are using a quill stem it gives you an extra 8-10 cm of height. but this is just my humble opinion