I decide this on 3 criteria:

Function
Appearance or Esthetics
Comfort

With the pocket options that contemporary kilts like FK's and UK's come with there is little functional need for a sporran. As long as you can carry everything you need in the pockets you won't need a sporran.

Esthetically there is little need for a sporran as well. Traditional kilts are very long and it can look odd having such a long portion of the body covered in tartan or solid cloth without something to break up the length. Traditionally that is done by wearing a wide belt and/or in conjunction with a sporran. These accessories bring the outfit and the overall image back in to a proportion that is attractive. Contemporary kilts are typically a little shorter than traditional kilts so there is less need to address proportions. Even if you did need a little extra capacity, a backpack might be a better choice than a tasseled sporran.

Comfort is my last criteria. That's completely preference but if function and appearance of my outfit are both satisfied, I'll opt against a sporran most of the time.