If in doubt, put one on any build, unless you can't for some reason. My favourite quad hasn't got a cap on it, but I just live with it. At some point it will annoy me too much and I will replace it with one of the xt30 power leads with cap installed.

But for instance on my Meteor85, it is the difference between it rolling let a lot or very little (due to a glitchy gyro / ICM20689). When i would hover without touching roll/pitch, the quad would veer left a noticeable amount, with a cap it does it far less. It turned out this gyro was very susceptible to dirty power.