Depends how you use the lens, for play or for work.
The front element is actually pretty hardy and can take a beating, even if you scratch it, photos won't show difference, but most of you wanna sell the lens after the fun is over, so maybe you wanna protect it.
Some environmental conditions is also good to have a filter on, ie oil and water spraying into your lens, hairspray, colored powder, firework sparks, saliva and slime, volcanic ash, etc, etc.
Use common sense.