If you're adopting a relatively formal written style, especially an academic style, it makes sense to choose to include that. But it is not required by the rules of grammar.
In examples like yours, that is a subordinator marking a content clause. It lets the reader know that the clause they're reading is subordinate and not a main clause.
Most of the time, that can be omitted from content clauses. That's because the reader can figure out from the structure of the sentence whether it's a subordinate clause or not.
In your example it's fine either way:
1a. Assume [ that you are given a ball ].
1b. Assume [
that you are given a ball ].
The reader knows assume must be the main clause verb, so you are given a ball must be a subordinate clause. There's no other possibility, so that is an unnecessary signal.
However, if the subordinator that comes before the main clause verb, it cannot be omitted:
2a. [ That you are given a ball ] can be safely assumed.
That you are given a ball ] can be safely assumed.
At the beginning of a sentence, you need that to let the reader know they're reading a subordinate clause and not a main clause. As a result, sentence 2b is ungrammatical.