For many years, cuers were instructed that the spoken English vowel /iēee/ when unstressed changes to /ɪĭi/. According to this reasoning, the final syllable of the word happy was cued at the throat: /hhhæăaˈˈˈpppɪĭi/. Instructors commonly told students that when words ending in unstressed /iēee/ (e.g., candy, cookie, trolley) are spoken naturally in sentences, the final vowel was more likely to resemble pit than Pete. After much debate and some division in the community, the consensus is that this pronunciation does not apply to many American English speakers. The word happy can be cued at the mouth.
There are many vowels that can be stressed or unstressed while not changing to another vowel. The word logo offers such an example. The vowel in each syllable is essentially the same – /oʊōoh/. But the stress is different. The first syllable contains a stressed /oʊōoh/ and the second syllable contained an unstressed /oʊōoh/. The two are differentiated by head thrust. When discussing a word like sleepy, instructors historically said that "unsressed /iēee/" should be cued as /ɪĭi/. However, the fact the they call it "unstressed /iēee/" instead of /ɪĭi/ is somewhat telling. If the vowel is truly just an unstressed version of /iēee/, it should be cued at the mouth without headthrust.
Unstressed prefixes can change /ɪĭi/ or /əəə/. Consider words like beneath. When the second syllable is stressed, the first syllable is likely to be pronounced as /bbbɪĭi/ or /əəə/. As always, this depends on dialect. However, in many dialects, prefixes like pre-, be-, de-, re- may use /ɪĭi/ or /əəə/ when unstressed.