houzi, in general, perennials store nutrients in the roots in preparation for winter; so you want the root system to be well formed and ready for this purpose in the fall (I should say 'autumn' out of deference to you).
The new flush of top growth occurs as the weather warms up in the spring. I would think that the time for root pruning is sometime between these two events; exactly where I am unable to say. Maybe this site will help.
http://www.rhodoniagara.org/feed.htm

Houzi - Rhodies are best repotted just before or just after bloom. If you repot before the bloom period, remove all flower buds to avoid putting too much strain on the plant.

Al

Oh thanks for the quick replies guys.I'll wait and see how many buds make it.If only a few I'll repot before bloom time,if looks like a good show I'll do it after.

Oh and interesting link...highlighting alternating top&bottom growth...I've learnt yet another thing cheers