I don't trim. The only thing I do is twist off the outer layer if the spurs concern me. There's a softer spur underneath that outer layer, and the softer spur will wear down a little over the following few days, ending up with a shorter spur. The softer spur slowly grows a harder layer in the next week or so. I haven't done it if the spurs are only an inch long. But, of course, it's on a case by case basis.
I have had one rooster that had spurs that curved up a bit and I was afraid he would impale himself when jumping down from the roost.
There is not precise length at which someone is supposed to trim their roosters' spurs, although some folks may have a preferred length. For the most part, for me, it's just making work for myself and troubling a rooster that is just fine with the spurs the way they are ... well, unless he's tripping over himself, which I've not ever seen ... yet.
