I am going to disagree a bit and say it completely depends on the situation. However, I am more of a use Cait ult whenever it is up on the ADC (if in lane) or other squishy if grouping up for siege or defense.
The reason is that Cait gets much more auto attack dependent as the game goes on. So much so that eventually you would do more damage just auto attacking than waiting for your ult to go off in a fight. So then, if you hold your ult you can ONLY use it "IF" your team wins and 1 or 2 guys are running away. In that case it is ONLY helping get a kill in a fight you already won.
If you were to use it in the middle of a fight it is wasted because you can do more damage simply auto attacking and that can cause you to lose the fight. Thus it is only 100% beneficial if used prior to the fight.
In lane (even in early levels) the same principal applies. If you use it BEFORE the fight starts you get to use the damage in that fight. As in, the adc (hopefully) that you hit is now lower HP than you to start and you can win with Auto's and Q's. However, if you were to try and use it mid fight, the fact that you have to sit there and line it up for a few seconds just leaves you open to get killed. And if they are running away and you use it, then you won the fight without it (but you may secure a kill). But it had no effect on you winning or losing the fight.
So if you are strong enough to win without it and just want to secure kills, save it. If you are close in lvl/items/damage then I would use it to start the trade so You begin ahead. Otherwise it is wasted.
My thoughts.
edit: I mean use when you can hit the adc or squishy for sure and it cannot be blocked prior to fight. So if I see a open window to get the damage in.