It depends.

For non-indexed computed columns, if the data modifies often and is not queried often, you should not persist it.

If the data seldom modifies and the persisted column is often queried, you should persist it.

If the data seldom modifies, the persisted column is sometimes queried and the other columns are often modified, it's an edge case. You save on evaluatig the expression when reading the computed column, but the extra bytes in each row slow down all accesses to the table.

EDIT: Removed a line where I erroneously claimed that persisting is required to index a computeed column. See the post by Henrik that follows this one.