No, they aren't a matter of taste any more than periods are. A period goes at the end of a sentence. A comma separates lists and independent clauses. I may have missed a correct one, but I believe all fifteen of yours are simply wrong and are extremely annoying.
That last sentence is an example of independent clauses - each side of the comma is a complete sentence by itself by dropping the conjunction:
I may have missed a correct one
.
I believe all fifteen of yours are simply wrong and are extremely annoying.
The independent clause can also be separated from a dependent clause:
At 10 o'clock, you overused commas.
The independent clause, which could be a sentence unto itself, comes after the comma:
You overused commas.
Another example:
A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed.
The independent clause after the comma is a complete sentence:
The right of the people to keep and bear arms shall not be infringed.
Lastly, there is the parenthetical, a dependent clause embedded in the middle of an independent clause:
The President, Barak Obama, refused to submit a budget.
Taking only the part separated by commas, we still have a complete sentence:
The President refused to submit a budget.
See:
http://grammar.ccc.commnet.edu/grammar/commas.htm