When should we...? Well, when you're doing an accent which uses glottal stops. It isn't used in standard English. Would you have an audio link for your US examples? I don't know exactly what you mean.

I disagree. Glottal stops are used in standard English. "Glottalization is a general term for any articulation involving a simultaneous constriction, especially a glottal stop. In English, glottal stops are often used in this way to reinforce a voiceless plosive at the end of a word, as in what?" (David Crystal, A Dictionary of Linguistics and Phonetics. Blackwell, 1997) •"We often make this stop--it's the sound we make when we say 'uh-oh.' In some languages, this is a separate consonant sound, but in English we often use it with d, t, k, g, b or p when one of those sounds happens at the end of a word or syllable. . . . We close the vocal cords very sharply and make the air stop for just a moment. We don't let the air escape. "This glottal stop is the last sound of these words: •words: light . . . flight . . . put . . . take . . . make . . . trip . . . report •multisyllable words: stoplight . . . apartment . . . backseat . . . assortment . . . workload . . . upbeat •phrases: right now . . . talk back . . . cook the books . . . hate mail . . . fax machine . . . back-breaking You also hear it in words and syllables that end in t + a vowel + n. We don't say the vowel at all, so we say the t + n: button . . . cotton . . . kitten . . . Clinton . . . continent . . . forgotten . . . sentence." (Charlsie Childs, Improve Your American English Accent. McGraw Hill, 2004) •"Nowadays younger speakers of many forms of British English have glottal stops at the ends of words such as cap, cat, and back. A generation or so ago speakers of BBC English would have regarded such a pronunciation as improper, almost as bad as producing a glottal stop between vowels in the London Cockney pronunciation of butter . . .. In America nearly everybody has a glottal stop in button and bitten . . .." (Peter Ladefoged, Vowels and Consonants: An Introduction to the Sounds of Languages, Vol. 1, 2nd ed. Blackwell, 2005)