Thanks for your comments, John. . . I honestly do not know the OP's actual reason for reaming because he did not state it.
But I emphasized that the reasons I gave were MY reasons. . . I have long been wary of the usual "pressing in" process.
"Pressing in" expands the brass tube . . . not by much, but sometimes enough to crack the blank as I found out very early in my "career".
Also, to repeat, I sometimes like to "get the look of" the finished pen before final assembly . . . I do a trial assembly after reaming to check the appearance and the constraints that may arise from a desire to align the blank with features of the hardware ( including position of pen clip).
So I often do the reaming for alignment reasons . . . After checking appearance, I can disassemble very easily and then do the final finishing of the blank before the final assembly with glue.
Reaming does not remove very much of the brass tube and it does not take much effort to ream out about 3 or 4 thou. on the radius.
I think that brass tubes are typically about 10 - 12 thou in wall thickness. . . You do have to ream with care and to not overdo it.
As I recall, the 3 or 4 thou is about the amount that the pen parts are oversized with respect to the internal dimension (radius ?) of the tube.