Agree with
@Wires_Guy_wires
I normally repot junipers after I've finished all the deciduous so that's often when they have active growth. No problems with that down here. Mine go straight back to the benches in full sun as it is generally still not too hot or dry through spring.
We see lots of advice to retain growing tips to help grow new roots when collecting junipers so it makes sense to me that repotting junipers with growing tips should be good.
Younger and smaller juniper (and plants in general) will tolerate much more than older trees so you have a much wider window for young plants than older bonsai.
BTW. best to add a location to your profile as many bonsai activities and timing depends on where you live. A city or general location helps others give better advice.