[elRey]

how important is it to ream the headtube when installing a brand new headset on a brand new frame?
could i get away with bypassing this step?

 

[sanjuro]

Is there paint in the headtube? You still push the cups in and hope for the best....

 

[w00dy]

If it's a low quality steel cup-and-ball headset, it doesn't matter that much.
If it's a high precision King-esque headset, you'll want to ream it so you don't explode bearings.