The same question came up when I was first learning Latin, and in spite of some sources which recommended otherwise, I’ve since made it my habit to pronounce an unaccented ‘i’ or ‘u’ (usually regardless of length) followed by another vowel as a glide (transcribed with ‘j’ or ‘w’ in the IPA). Somehow my knowledge of modern Romance made it very difficult to believe that the Romans didn’t use such glides (even though I had much less trouble getting used to pronouncing, for example, ‘v’ as ‘w’ and ‘c’ as ‘k’ even when followed by ‘e’ or ‘i’). So, what’s the stance on this in classical Attic Greek?
I vote for yes. I’m not an expert on pronunciation issues, but it seems absolutely natural to me.
The modern pronunciation in many countries of many Greek names beginning with iota + vowel does this (Iason = Yason), but I don’t know of evidence that this happened in ancient Greek. The place to check this would of course be a work such as Allen’s Vox Graeca
Correct me if I’m wrong, but a glide would not function as an independent syllable, correct? In that case, it should be easy to tell whether iota is functioning as a glide or not in most cases – look at accentuation and syllabification. If unstressed iota consistently has the status of a full syllable for metrical purposes (and as far as I know it does), that suggests that (at least in an early period, in the formal setting of poetry) it did not function as a glide.