I'm still trying to find a satisfactory answer to my question, but it seems as if even "Mr. Google" can't help me. It's hard to believe that there is no binding rule in this particular case. :-(

Since even the most comprehensive grammar books and style guides cannot tell me whether the adverb "either" should be preceded by a comma or not, I've come to the conclusion that there is no rule in this particular case.

Just a thought - when the clause is negative, 'neither' is to be prefered to 'either'
I haven't seen the movie and my brother hasn't either.
I haven't seen the movie and neither has my brother.
I haven't seen the movie, neither has my brother.
Neither I nor my brother have seen the movie.
'I can't swim' 'I can't either'
'I can't swim' 'Neither can I'/'Nor me'
I was not aware that 'either' was always preceded by a comma - Either I'm right or I'm wrong.- He is wanted either dead or alive.- If he is not at home, he is either at the football match or in the pub.
Punctuation rules are not as strict in English as in German, at least mine aren't.If there are rules, I'm not an expert in them. I put a comma where I would pause for breath or ending a phrase or thought.
as above - as in German(,)at least....If he is not at home(,)....
If there are rules(,)......

I think it is strange that some lexicographers and grammarians put a comma before the adverb "either", whereas others do not use a comma at all here (please see the example sentences in my first post).

My personal conclusion:
(1) There is a rule, but I'm not aware of it.
or
(2) There is no rule, so that I can decide it for myself when the adverb "either" should be preceded by a comma.

I would like to know how other native speakers deal with this tricky adverb:
Do you SOMETIMES put a comma before "either"?
Do you NEVER put a comma before "either"? or
Do you ALWAYS put a comma before "either"?

Look at your examples and you will find that a comma in front of 'either' _only_ is found when 'either' is the last word of the sentence and when it emphasizes the content of the otherwise complete sentence.

So the answer to your last question is "sometimes".

Yes, this makes sense. Thanks for your reply, AGB.

You have the choice of two colours,red or blue. You can choose either red or blue,nothing else.
You have the choice of two colours, either red or blue.

There is a comma in each of the three sentences. The comma in the third is not there because it precedes 'either', it is there for the same reason it is in the other two - there is a slight change of thought or more information. Note also, a comma would not be placed between 'either' and the verb - see sentence2.

It is not easy to say when a comma should be used, but clearly it does not always come before 'either' and, when it does, it is possibly for some other reason. The answer is,as AGB says, sometimes a comma comes before 'either'.

JGMcl, you are right when you are taking about "either" used as a *conjuction* (either ... or). However, my question is when do you put a comma before "either" when this word is used as an *adverb*. Nevertheless, thanks for your reply. :-)

I learned at school (chiefly 1980s) that in sentences where "too" or "either" is the last word, it is separated by a comma:

"I haven't got a clue why it fails." -- "I have no idea, either".
"I have been to Paris last summer." -- "Oh really, I have been there, too."

However in practice I have noticed that this rule is almost never actually followed by native speakers.

>>However in practice I have noticed that this rule is almost never actually followed by native speakers.<<

The thing is that the two sentences I mentioned above were taken from the same dictionary. The lexicographers put a comma before "either" in the second example sentence, whereas they put no comma before this adverb in the first sentence. Consequently, I thought there must be a reason for this (arbitrary??) usage.