How does the practice of Buddhism look at preconceived ideas, assumptions or notions?
Preconceived ideas, assumptions, notions and habits are what we try to understand and do away with in Buddhism.
Through the practice of the Noble Eightfold Path we come to understand the Four Noble Truths. We gain Samma Ditthi - Right View, i.e. the understanding of the four noble truths and the true nature of reality.
When practicing insight meditation one is observing ones mind with detachment, objectivity, non-judgement and non-interfering. That allows the mental and physical phenomena to show themselves, to reveal there true nature. They are allowed to "speak" for themselves without us extrapolating on them.
When not being mindful and not guarding the sense doors, the third and fifth aggregate, i.e. Sanna and Vinyana creates misconceptions and extrapolates on phenomena.
Paticca Sammupada - The doctrine of dependent origination shows how our misconceptions of reality arise. How we begin to like or dislike phenomena because we have wrong view. We think phenomena are permanent, satisfying and controllable and therefore we want them, we cling to them or we dont want them and try to get away from them.
When guarding the sense doors one is not falling into delusion. Instead one is seeing reality for what it is, i.e. the 3 signs of existence: Anicca, Dukkha, Anatta.
When one is practicing insight meditation one sees for oneself how conditioned habits, ideas and notions arise. Things that have been build up over a long period of time. In a way we learn to "unlearn". When practicing this path we shed our delusional shell. We do away with illusions about reality. Things go from complex to very simple.
When practice gets more advanced one begins to see non-self, i.e. that phenomena are uncontrollable, ungovernable and thereby oppressing. One sees that there is no substantiality, no inner-core, no permanent self, no experiencing entity. Instead one sees that there is only phenomena arising and ceasing and mind arising to these phenomena. Thats all there is.
Preconceived ideas, assumptions and notions are conditioned and a product of having wrong view. It's like a scientist having a theory about gravity. When he is only intellectually interacting with the theory he will have no real point of reference and therefore he does not know whether or not his theory is correct.
He then decides to test his theory on reality and then finds out that his theory was not correct. He then got a point of reference. He has done away with wrong view and have now seen reality.
This is example is not the best one but i hope it gives an idea of what we are talking about here. Before practicing insight meditation one might believe that conditioned phenomena will make one happy. After having practiced insight meditation and gotten some insight into how reality functions and one has seen that phenomena are impermanent one come to the conclusion that conditioned phenomena are not lasting and therefore not satisfying. One has then achieved a greater deal of Right View.
So to sum up. Preconceived ideas, habits, notions and assumptions cannot help us if they are blocking us from seeing the true nature of reality. The true nature of reality will reveal itself when practicing insight meditation and being mindful of phenomena. Then we come to develop right view and naturally wrong views fall away.