Well I do because they have the irritating habit of producing new leaves at the end of the flowers; when the flowers fade, it leaves a bare patch on the stem where it's been, with the new leaves on the end continuing to grow, so I clip them off just behind the bottom of the flower when the flower starts to fade.

I've never bothered. You get the odd seedling to pot up for sale.

I've only done it when I wanted to control the size, and make it more compact.

Hmm yea, I should have said, mine's in a pot on my balcony, which is why I'm so fussy about those bare patches on the stems, I'm guessing I probably wouldn't notice them if the plant was larger and in the ground.