Is a certain amount of shagginess 'cool' in a garden?
Cool meaning - 'hot' - of course, and 'hot' meaning popular. All I know is that the naturalistic look is 'in'. And naturalistic means a certain amount of shagginess.
Probably not acceptable, if you are a neat and tidy gardener though - it depends on what you value most about your garden.
Clipped edges and hedges or a sense of abundance spilling over ?
I am not such a neat and tidy gardener that I need my garden borders to be regimented in neat rows, or to be sharply trimmed. And yet, even though I do like abundance to the point of chaos, I also like a little bit of shape, so that you can tell that the garden actually is a garden.
Farmers would throw their hands up in horror at the thought of growing Muehlenbeckia complexa as a decorative plant, because even though it is a native, it's all too common in the countryside, as a nasty twiner which invades fences, gates and anything else it can find to smother. It also sends robust runners under the ground which pop up everywhere. As well as being super shaggy above ground!
So like it or not, I simply do have to clip and contain this most invasive of plants
As for Santolina, it's at it's best when unclipped and billowing about. But when not clipped, it becomes woody, leggy and out of shape.
Like it or not, it appears I do have to do a certain amount of clipping even just to keep the abundant tumbling look I prefer! Because if I didn't, I probably wouldn't have a garden at all, but an unruly wilderness. I wouldn't really like to see M. complexa smothering everything, as there would be no colour or shape left in the garden. Or would I like to see soft feathery Santolina growing into a tangled woody mess.
So it seems that gardeners are often pulled in a myriad of different directions - if you clip the garden too much it will become too formal and and rigid and not billow about enough. If you don't clip it enough, it will become out of control and you feel you won't be able to manage it