@dega2010: Generally, most sleeve tattoos are pre-planned just because it helps the flow of the sleeve. However, this isn’t how it has to be.
You can start a tattoo on your arm with no intentions of making it a sleeve. Then, over time, you can add more tattoos. Eventually, it will turn into a collaboration of either random tattoos or a collection, whichever you choose. Sometimes,the clashing of different tattoos may not go as well, depending on the size of the tattoo and things such as that.
Both adding as you go and pre-planning are both common ideas used today.