Sure arnoldt, there will always be differing opinions, but I think what most people are saying it to keep the choice within the accepted boundaries of time and space. No science fiction "visitors from the future", no mysterious time machines, no worm holes, etc.
Translation: 1975's Giant-Size X-Men 1, while an important book, could never have started the Bronze Age simply due to its late publication date. Unless of course, you only want Ms. Marvel, Nova, Devil Dinosaur and ROM as your top Bronze Age books/characters.