You'll read a lot about working on and wiring Juniper in the autumn, but there are rarely any reasons given.
It might be that the tree's metabolism has slowed and there is less demand on the roots for moisture? There is a little bit of rationale on the BCI page.
http://www.bonsai-bci.com/species/blue-juniper.html
One reason I can see is that many confers take a long time for the branches to take a set - so if you want to maximize the time before you might need to remove the wire to accomodate summer growth, you'd wire in the autumn.
If you are careful, you should be able to wire your conifers whenever you have the inclination - but if it is during a period of active growth, do be especially careful about the possibility of developing wiring scars - so you might have to wire them again to get the affect you want.
Regards,
Matt