Hi hardy,
Now should be fine providing there is no likelyhood of frost forcast.
As you may be aware they are acid loving plants, so prepare the planting hole well and add plenty of ericacious compost and organic matter, Never use mushroom compost as this will raise the ph level.
They are shallow rooted and care must be taken to plant at the same depth, as they where growing in the pot and try to avoid any root disturbance when transplanting.
A thick layer of mulch will help them settle and give them plenty of water during dry weather (rain water preferably!)
If the leaves start to turn yellow, water with sequestrene to acidify the soil. they will do best in dappled shade.
The BBC gardening has some very good tips on rhodo's.
Good Luck Tbird+