Hai Sakura8 - Im waiting on my order of plant tweezers :( Its hard to get them here. I was even looking at getting surgical equipment since its easier! Fortunately found someone who will import it for me.
As for the scissors - I use normal ones and I would give my eye teeth for the bent one. It makes trimming stems so much easier when you have a bushy plant. Most people I know use both. The bent one is used to trim deeper in the plants and the straighter edged one is used for trimming plants from the top down or mosses.
I would go on growth - I trim now about once a month and then I trim about 10 - 15cms off some of my plants! Trimming is a personal choice since it helps to create the look you want to achieve with your plants.
This is my tank after trimming. I prefer a more cluttered/wild look so my plants are not perfectly shaped or anything like that. I tend to trim when the growth gets out of hand (aka my fish doesnt have anywhere to swim - I kid - I just do it monthly since I see hectic growth in that time).
The Indian Ferns (aka watersprite) was one big bush that covered the whole corner there and I thinned it out and only kept some of it. Before I trim I normally make a post on our local forums to find out if anyone wants any of the plants. If they dont I trim and take the cuttings to my LFS or make compost out of it. Since we are very careful about releasing plants into our water systems - we cant even have a small piece such as a leaf from the watersprite end up in the drainage systems here in case it takes over - I tend to throw my cuttings on the compost or give it away if I am not going to use it.