I find it personally better when you use the same item for similar reasons, such as one for protection, one for luck, one for attracting, one for banishing, etc. If you change it, you should clear all the energy from the item and then you need to start from the beginning again--charging it with energy, giving it instructions, etc.
I used to use the same gemstone for various reasons when I was young, but I found them to be stronger when I reinforced the same purpose everytime and added more energy to it each time. Now I never clear my magickal items. I only cleanse them and then charge them with the same energy.
Of course, I used the same ones for different purposes because I was limited to what I had. So I think it all depends on what is available to you. But if you have the ability to have individual items, each used for a different job, it is worth it.