I would observe the trim tabs from behind the boat as someone trims them fully down and then up. Note the angle when all the way up. They should be less than a 90* angle, so they do not influence the trim of the boat.
Now in practice, trim fully down, motor trim down and accelerate to plane. Experiment to see what gives best planing action. Trim fully up when on plane and observe speed changes. Trim down at cruise speed and observe changes to boat trim and speed.
Trim tabs down are good for planing a full boat of people/equipment, staying on plane at a lower speed and keeping the bow down for better ride in rough water. I usually do not use them for other purposes.
They are especially useful in rough water. Put 'em down and back off the throttle until you are just on plane. That gives the best ride with some planing speed.
Also, I could pull a skier in my 18 footer (with I/O engine) with 6 people aboard. I needed full tabs down to plane and partial tabs down to cruise. The boat handled like a barge, but without the tabs it would not have worked at all.
You can balance the boat side-to-side using the tabs, however, I prefer to shift weight if I can, to do that.