Balloon placement is composition
The letterer is the last person to change the picture, and always does.

The letterer changes the picture last
Every balloon added to a finished page is a decision about what the reader sees — and when. The letterer arrives after the penciller, after the inker, often after the colourist, and begins placing objects in a drawing that never quite left room for them. Sometimes leaving room was planned; often it was not. Either way, the balloon lands where it lands, and something underneath it disappears.
That disappearance is not neutral. A balloon in the upper-left corner of a panel pulls the eye there first — which is where the eye was already going. Put it upper-right and you have created a small argument with reading order. Cover a character's hands during an emotional beat and you have cut gesture from a scene that needed it. These are compositional acts, even when they feel like logistics.
Placement also controls sequence within a panel. Two balloons in the same frame imply order: left before right, top before bottom, with the tail of each pointing to a mouth. Where the tails go sets the rhythm of the exchange, and the letterer is conducting it. A balloon placed so its tail crosses another balloon's tail makes both of them ambiguous — the reader must pause to decode who is speaking, and the pause costs pace.

Size matters just as much as position. A large balloon occupies visual weight. It reads as emphasis the same way a solid black area does: the eye settles there. A balloon bigger than the face it belongs to has begun to compete with the drawing. This is occasionally right — a shout, a speech that dominates a character — and usually wrong.
The best letterers work the same way a good editor works: they find what the page is already doing and reinforce it rather than fighting it. When a panel is composed with open sky, the balloon goes into the sky. When a face is the panel's centre of gravity, the balloon orbits it without obscuring it. When the panel is dense and there is genuinely no room, the letterer tells the writer — or quietly reduces the word count, which is also a compositional decision.
The letterer is the last person to change the picture, and always does. The question is only whether the change was deliberate.
