I have heard of a few guys changing their Junkers axle to a brass tube which does have merit. I built the WnW Junkers J.1 when it was first released a few years ago and built the model using the kit supplied axle, to date I have not had an issue with the axle breaking or bending, I just now checked the model and the axle is still nice and straight. So, replacing the plastic axle with brass tube might be adding a bit of security but I feel that it is unnecessary, again, the choice is yours.
Des.