Where do trees get their shape?
Dr Karl Niklas from the Cornell University has written a book about the effects of factors that influenced the shape of early trees. He created a computer model in which he drew trees in the shape that favored particular characteristics.


  When Niklas favored plants that could gather light from the sun, they
  had non-overlapping horizontal branches. When reproductive success was
  favored, the plants grew tall, in order to spread their seeds. When
  the trait selected for was mechanical stability, the plants had few
  horizontal branches.


When he combined these three favourable traits, he noticed that most of the trees today are in a form that favors these three traits (reference). 



L is light favourability, M is mechanical stability and R is the shape of trees that favor reproductive ability.

Explanations for a few shapes found commonly with examples can be found in this article. 

Illustration for the common tree shapes Columnar, Pyramidal,Vase, Round, Spreading, Weeping and umbrella with a few more examples can be found in this article (reference). 

Another interesting thing about trees is the way in which their shapes can be influenced for the purpose of landscaping (reference).

What you mentioned about pine cones in isolation having no definite shape at all, I would like to see a reference that this is a fact before I can elucidate on it further (I couldn't find any articles that mentioned such a phenomenon). 

The reason you felt so could be because it were two completely different species of pines. Counting varieties and subspecies, the plant list of the Royal Botanic Gardens, Kew and Missouri Botanical Garden accepts 175 names of pines as current(reference). Here is a list of different pine species (reference). 

Here is an article on how tree shape and light available influence yield in cherry trees.