The assignments of the Kyriale are not obligatory, but within reason and custom. I know of one apostolate that, because the Incarnation and Our Lady are linked (the doxology of hymns for Marian feasts is that of the incarnation), uses Mass IX for Christmas and its octave.

My inclination would be that it is best used when Mass is of the season, until the week of the Ascension, just like the Benedicamus Domino of the Divine Office is of the season, including on ferias of Paschaltide, but of the feast otherwise.

On Chris's note, beginning this month, we've moved to Mass IV for these special feasts on Sunday and during the week (except those of Our Lady), including the solemn feasts of the Lord, since we cannot justify Mass II at this point (or ever? to be determined).

I made up this chart of the different seasons and feasts, following the pre-1955 divisions and rubrics, following the groupings found in the modern editions of the Kyriale with some loose inspiration from the (trad?) houses of the Solesmes congregation. You could easily plug in Mass III instead of IV, for the I class feasts not of Our Lord, and some other Mass instead of VIII for minor feasts, if you were even more ambitious, and this is already quite ambitious, but generally, I think that this would be really good. It basically assumes that you'll never do polyphonic ordinaries, and I'm mildly against them to a certain extent, but I also know that, even if I weren't, some places will never be able to maintain even an all-volunteer choir, whereas they will always have a Gregorian schola…

In any case, that extra ambition, for lack of a better word, would be good. The interruption of the Ascension means that while the season is still of Paschaltide, perhaps the Gregorian ordinaries through the week of Pentecost should be something other than Mass I.