Newbie Questions On Corks, etc. ?

Hello,

New to wines, but trying to learn.

I've read that wine bottles should always be stored horizontally.
I guess that this is to keep the cork (if a natural cork) always wet.
a.  Are there other reasons also ?

b.  What about if the bottle came with a plastic cork substitute ?  Horiz.

still desireable ?

And,

c.  Once opened, and the natural cork is replaced by one of those plastic 
cam-expansion caps, is it still necessary or desireable to store the
bottle 
horizontally ?

Why ?

Thanks,
Bob