This "Chelsea chop" thing! This is something I instinctly did years ago starting when I got fed up with constant dead heading of some perennials. So I,chopped it back and welcomed a rejuvenated plant a few weeks later. The Chelsea chop is simply cutting back plants,that you know will be leggy come late summer. By cutting back, maybe by half, in May or early June, the plants become sturdier and shorter and only a little later in flowering. I do this with annuals too,,such as cosmos which are vigorous tallish plants in mid summer onwards. Cutting and pinching these back before mid summer creates much better garden performers that need no support.
Waterbutts, there is no risk to the plants ....just decide which plants will get leggy and cut them back before the end of June.