That is not really making any difference if you can prepare beforehand

and

you can’t be sure which things can get you in trouble.

It does make a difference: you only mark enough to be kept to make your phone looks like it's in use, as opposed to obviously wiped.

By default, new material will be deleted. So you don't have to prepare again and again.

> you can’t be sure which things can get you in trouble.

When in doubt, don't mark it as keep.