I've read somewhere that if the first syllable of a word is heavy (i.e. has CVC structure or has a long vowel), then it gets the stress. However, younger speakers of Seoul Korean don't make the long vowel distinction. Does that carry over to the determination of the location of the stress?
According to (emphasis added):
Though researchers agree that Seoul Korean does not have lexical stress, it is controversial whether Korean has fixed stress at the word level or phrasal stress. Some believe that Korean has word level stress and that it is sensitive to syllable weight (H.-B. Lee 1964 , 1974 ; H.-Y. Lee 1990 ; see Lim 2001 for a review); i.e., a word-initial heavy syllable is stressed, and if the first syllable is not heavy, the second syllable is stressed. Thus, according to this view, stress falls on the initial, i.e., first or second, syllable of a word regardless of the word length. Here, `heavy' is defined as a closed (CVC) or long syllable and is claimed to be acoustically realized with a longer duration.
However, production and perception studies of stress in Korean (Jun 1995a; Lim 2001) suggest that the perception of stress on the word-initial syllable is due to the intonation pattern of Korean. Jun (1995a) showed that the so-called `stressed' syllables are always realized with the fundamental frequency (f0) peak when the word is uttered in isolation.
So what you said is quite not true. Seoul Korean intonation patterns are beyond complicated than just "stressing the heavy syllable", and it depends on a lot of things. For more info, check the above paper.