You can check this dictionary here that offers the phonetic transcription and it's usually quite accurate. The pronunciation adopted is the "standard" one and for this specific case, just like Carfer says, both are correct.
One day when I was crossing Praça Martim Moniz (longing for a ride in the elétrico 28), a senior citizen asked me (in PT) "what time is it", to which I answered (in PT) with three, maybe four words. He gave me his thanks quite politely and added (in PT) "you are not local, are you"?
No, I was not, I am not. Notwithstanding that, I speak Portuguese from the very cradle, and it´s been a number of years since that cradle, more years than I like to count. Moreover, while I don´t claim to master my mother language completely - I don't believe anyone does -, I consider my language skills to be above average, at least.
Even though, the man could pick, amongst no more than three or four words, the alien accent.
The difference between a full "e'_" and an "e" schwa (notice that I´m going even further than the i'_), if anyone notices it, will be as important as my pronunciation of the hours. That's to say nothing in terms of intelligibility. Something in terms of accent.
As I am not "local" not even to this place in Brasil where I live... actually, and to be fair, I am not considered local to the place where I was born, since my accent has changed through time... what is realistically the level of similarity that we are pursuing for second languages?
My received pronunciation of "herói" in European Portuguese would be [i.ɾɔj] due to the normal and standard vowel reduction found in that variant, even if pre-tonic. I'm a native speaker currently learning Portuguese at the university and since it's not either in Brazil or Portugal, it's taught as second language. My Portuguese teachers, the Portuguese ones, usually recommend that you always reduce vowels whenever possible; doing so you're less likely to mispronounce most of words. When European Portuguese is concerned, usually if a word doesn't follow the vowel reduction pattern, there's a reason for that (Brazilians are more likely to neutralize). For instance "você", which has a [ɔ] in EP instead of the expected /u/ — reason for that is the word it stems from: "Vossa Mercê". However, there is no reason for "herói" to be pronounced with [e], it's likely just an independent evolution of that dialect, which probably contrasts with the rest of the country, as you can read above in #6. There are other equally greek words which carry the Portuguese vowel reduction, differing from the variation found in Lisbon. It seems to be an independent case without a clear explanation and a case which has a free variation or at least has some pattern; see above #2 and #7. Note, however, that the Dicionário Fonético only lists [i.ɾɔj] as standard and Infopédia lists it first in contrast with [e.iɾɔj], cause it follows the vowel reduction rule and it's very likely to be more common nationwide and even in the capital.
There doesn't seem to be any article or study that measures how often the variation occurs in Lisbon, so it is probably free.
I don't know how far gone you're in the language, but apparently far enough to care about the difference, but if it's still not that important to pronounce everything strictly as the people from Lisbon — or any other accent — do, just stick with the vowel reduction and you'll be fine. Portuguese learners who try to stick to the more standard variations (from Lisbon and Rio, due to the media) are often left in the dark with rules cause those are the dialects with the most eccentric innovations and whose innovations are sometimes hard to keep up with rules.