well,

Since the only evidence we have of any dinosaur is from fossilized bones, I would say that this is conjecture on your part. I happen to agree with you, but we do not know for sure. A lot of things about dinosaurs that were supposedly "sure things" have changed pretty drastically over the years... like warm blooded or cold blooded, whether a saurapod could really live outside of a swamp, the intelligence, related to reptiles or birds, and on and on. I don't think we know enough about Brachiosaurus to know if she chewed her food or not.

Wear Marks on Teeth, and Tooth Shape

It's quite easy, actually, to tell something of the diet and eating style by looking at the teeth. Wide, flat teeth indicate MASHING, as in a pufferfish and a cow's teeth. If the teeth show a circular grinding pattern, it's obvious that the animal is capable of chewing.\rSpiky teeth, widely spaced peglike teeth, baleen (like in whales who filter-feed) or flat teeth which lack wear-patterns consistent with grinding would clearly indicate a lack of ability to grind and chew.\rLook at your own teeth. It's quite clear from shape alone, that your mouth is set up for both cutting AND grinding, but (unless you need braces) not for filter-feeding.

Teeth

The first commenter isn't being entirely accurate when he states, "the only evidence we have of any dinosaur is from fossilized bones." There are also trackways, eggshells, and coproliths. In any event, leaving shelled animals aside, the parts most likely to fossilize are, in fact, the teeth. As paleontologists trace evolutionary pathways, they are often following the results of one set of choppers mating with another.\r

The animal kingdom shows some interesting adaptations for eating. The pangolins, a toothless order of mammals, have developed bony knobs at the entrance to their stomachs. It is as if their teeth are not in their mouths, but in their bellies. Birds, relatives of the dinosaurs, are all quite toothless. Many, however, have the habit of swallowing bits of gravel which helps mash their food inside their gizzards.\r

As for Brachiosaurus (B. altithorax), with a set of rather peg-like teeth, while the consensus view is that of the nitpicker, it should be noted that Brachiosaurus skulls are not that common, and our view is based upon a limited sample.