During cycling, you get into the habit of testing the waters and you get a 'feel' for it. This habit should continue when you add fish.
The problem usually occurs when,at last free to add those much-desired fish, we can rush ahead and completely stock in the one go.
Yay!
It's even written in many otherwise esteemed places that you can 'now add all your fish'.
This is just asking for trouble and forgets the concept of the balanced ecosystem you've just taken the time to create.
In many instances, you might be able to add all your fish and everything is hunky dory...but why risk it? For sure, you can start again and 'cycle' a bit longer, but you've gone and damaged, if not killed, fish in the process.
If you have a planted tank and all of the plants die, then something is seriously amiss. If we took half as much care with our plants as we do our fish, then plants would survive longer and might even 'thrive'.
@Byron 's point about floating plants is well made and these are great, not only for what they do to the water chemistry, but for the security they give the wildlife in the tank.