Spontaneity isn't necessarily a J/P thing. Environment plays a large role in how this plays out in a particular life.
For example, J's instinctively do like to know what is going to happen next, but if they're brought up in an environment that is very free-form and yet nothing bad ever happens -- if they learn that they can trust their own instincts rather than needing to know things ahead of time -- they can end up being more flexible. Or, negatively, if a J is taught that her decisions don't matter because her plans are always overruled by larger social forces, she might just give up completely.
Likewise, P's might be taught the value of structure by good parents, and learn how to organize themselves in ways to be more effective and enjoy that. Or, negatively, P's who are constantly undermined, intruded upon, or constantly overruled when they try to act can become very demanding and require less be left to chance... so they feel they have some control over life.
In addition, MBTI type of P or type of J will impact the need and bent towards organization. TPs might organize a bit better (because of the impersonal T), FPs might organize a bit worse (because of the need to satisfy the immediate inner values). And so on.
Using myself as an example, I should be much more spontaneous than I am. I am very "open" ... but I have a very hard time just having someone change my plans or intrude at the last minute, because my boundaries were violated often when I was a child. Now sometimes I feel very inflexible, if I had my mind set on one thing and someone just changes it at the last minute; I can't flex. I also have intentions of creating to-do lists and like to develop mental ones -- I develop strategies by which to tackle my day -- but creating an actual to-do list on paper and following it is very hard for me.
I think I just mean to say that having a strategy says nothing about being J or P. SJ types do seem more likely to have a paper to-do list of concrete steps to carry out. NJ's less so, but they still do it. (I know INFJs do.)