You should always use kiai...
Kiai is more than yelling. The way my sensei explains it is this: ai means harmony and ki means energy. Aiki is harmony with an attacker's energy -- kiai is the flip side of that: harmony with our own, internal energy. Thus, kiai consists of all parts of the body being unified and directed to one intent.
For example, we almost never turn our trunk without also movoing the hips and legs. We don't break our structure by hunching our sholders or bending at the back. Our who body's structure is tuned to the particular movement we're executing. This also, of course, involved breathing, and that is where the yell comes in.
It's long been known that a good, relaxed, powerful exhalation adds power to movement. The yell is just an audiable indication of good kiai (aligned body structure, focused intent, and good breathing). It's not the yell that's important, it's the things that make a good yell that really count. My instructor says the yell is just a good way from him to tell how well we're doing.
I hope this helps,