In careful writing, I personally think (that) it is usually better to include optional "that". Exceptions might be when two or more occur in close proximity, or in certain particularly common idiomatic combinations. I especially dislike the omission of "that" when it leaves a "verb + object" grouping that can be misread.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.