I'd just like to note that overgeneralizing too much is really bad and you need to learn certain patterns.
Example: Cho'Gath top just fell behind really far. Luckily, your Cho'Gath isn't a retard and is rushing CDR tank so that he doesn't mercilessly feed. With his E, he'll be able to keep the minion wave off the tower (or at least not on it for extended periods of time) meaning that leaving him alone even when behind won't immediately lead to a lost objective. This is a situation where it is safe not to gank Cho'Gath if you think you can do something more important somewhere else.
Example: Yasuo mid has just barely fallen behind, but is holding in there. He is a stubborn butt though and is rushing damage, hoping to keep on his opponent's toes and outplay them. This is a lane you want to gank as soon as possible, because with Yasuo's current build, if he falls behind he'll fall into near uselessness but given his matchup, he's still a solid damage source if you were to gank his laner.
Example: Your ADC is fed as hell, but the enemy tower hasn't fallen. This is a situation where you can choose to gank and rush the objectives down, or let the lane sit for a while as your ADC scales harder and harder without too much risk of dying because they're fed.
Example: A heavy CC/Utility Champion, like most top lane tanks, gets fed. Follow them when they roam or cover their lane. Unless the enemy is already low, ganking them isn't the best guarantee for a kill. On the other hand, if they were to roam to another lane, their ganks are usually extra dangerous. Communicate with them so that they can carry the burden of a feeding team.
There are patterns to notice. Most dedicated carries want ganks before they fall behind too hard, but don't necessarily need ganks or view ganks as an extra luxury if they are ahead. Most dedicated utility tanks don't need ganks to recover from falling behind, but they tend to provide the most assistance when ahead. There are also others that are more specific (Jax for example is a weird champion in terms of your priorities ganking him), but the important thing is that you learn that there is a pattern to when you need to gank instead of this one big unified rule.