Taking a whole branch off is totally fine right before flower but there is one important thing: It should NOT be one of the 'tops'. So if you have a plant that was pinched, and there are 4 vigorous leads and then a few others lower, take the low ones. If you take a top one, give 2-3 weeks for the hormones to redistribute. The bummer is the high branches take off faster after rooting- again, because of hormone distribution.