Contrastive dependence (WHEREAS, WHILE)

I have received conflicting instruction about whether/when commas are required between oppositional phrases of contrastive dependence.

When a subordinating clause is used at the beginning of a sentence, obviously the opposing phrases must be separated by a comma:

- Whereas the penny was plated with copper, the nickel was plated with silver.
- While she looked ten years old, she was was actually fifteen.

But when a contrasting subordinating conjunction is placed between two clauses, does it always require a preceding comma?

- The nickel was plated with silver, whereas the penny was plated with copper.
- She was actually fifteen years old, while she looked only ten.
- Traditional robots run on acid batteries, whereas newly developed robots run on solar cells.
- I prefer white wine, whereas my boyfriend prefers beer.
- I am single, while my best friend is married.

I wonder whether the rule changes depending on the isolated independence of the contrasting clause? i.e. If the contrastive dependence clause is not complete (lacks subject/verb) then no comma is required before the subordinating conjunction?

- I ate all of the cheese, except I didn't touch the brie.
- I ate all of the cheese [ x ] except for the brie.

- Traditional robots run on acid batteries, whereas new models run on solar cells.

- Traditional robots run on acid batteries [ x ] whereas new models don't.

- I prefer white wine, while my boyfriend prefers beer.
- I prefer white wine [ x ] while my boyfriend prefers red.

Can somebody please clarify the use of punctuation with the subordinating conjunction whereas?

Thank you in advance.