Q:

Is it still considered correct to append an "e" in old-fashioned dative?

I recently watched some shows by Prof. Heinz Haber (1968) and I noticed, that he tends to append an "e" to the dative as in auf dem Monde or im Raume.
Udo Klinger lists the added "e" in parenthesis, which I interpret as "okay, but not necessary".
I suppose it is not okay to add an "e" if the noun ends with a vowel. But I am not sure. What are the rules when an extra "e" is okay and when it is not.
In contrast to the already answered question:
When, if at all, should I add an e to the end of a noun in the dative case?
I'd like to know when you can add an "e", not when you have to.

A:

The Dative-E is considered dated, but you can of course use it. According to Wikipedia, it never appears with:

feminine nouns
nouns ending on -el, -en, em, -er,
names or foreign words

Dudengrammatik lists four factors in § 317:

I) word is part of the basic vocabulary or part of an idiom
II) exalted speech
III) nouns which can have a long genitive ending (des Baumes)
IV) the nount has to be preceded by a declinated word, such as an article!

You should use it for certain idioms which contain dative case:

Im Zuge dessen
  Am Tage der Wiedervereinigung
  Nach dem Tode meines Mannes
  Im Jahre des Herrn

