based on your own judgment of the skill. this requires aigis experience and some thinking about it, as well as how you use the unit. for example, black samurai hibari's SAW is not preferred due to having a long cooldown, since her normal skill is more or less sufficient. Kikyo (black shaman) is also usually kept at normal skill due to 1s toggle into healer instead of 5s initial time.
using the tenhou example of bashira, quadra shot may not be preferred due to x0.9 attack during skill. there's also the skill uptime (duration/total time before next use), where total time = duration + cooldown/reuse time