In August, some of mine (one year from cuttings) were looking a bit tired and only thinly flowered. So I cut them back to form a crude dome perhaps 30cm across. All those are looking good now, though the new shoots didn't come entirely from where I expected - so there is still something of that dome persisting, which doesn't look great.
Of the ones I hadn't cut back since Nov13 (two years old), some are just bigger and bushier, and full of flower (ie. no harm done by not pruning); the others are dying back (all in the same area, including the one I posted about earlier, suspecting virus -
http://www.mgsforum.org/smf/index.php?topic=1577.0).
I suppose I'll prune again in November, though the plants seem more firmly anchored now - so less of a worry in the winter storms.
I do not have any self-sowings yet, which is a bit of a shame as I have some steep grassy banks where the addition of Gaura would look very good.
Still love 'em.