Metals will taper outward from the top face when cut on the waterjet.  This means that the overall size of the part will be larger on the bottom face than on the top face. Holes will be smaller on the bottom face than on the top face.
With plastics, it is the reverse. Plastics will taper inward from the top face when cut on the waterjet.  This means that the overall size of the part will be smaller on the bottom face than on the top face. Holes will be larger on the bottom face than on the top face.
Big Blue Saw's low-taper waterjet cutting service eliminates this taper for an extra charge.
You can see the effect in the photo below. Both parts are made from aluminum 6061, but the one on the right is made using a low-taper waterjet cutting process.

Low-taper waterjet cutting is almost never worth the extra expense for parts 1/8 inch (3 mm) thickness or below. The taper on parts this thin is much less pronounced than on thicker parts and is typically less important to the design anyway.

On the other hand, if you have thicker parts that make contact with other pieces in an assembly along their cut edge (rather than just on the face), you should strongly consider low-taper waterjet cutting.

Illustration : Two parts cut from 3/4 inch thick 6061 alloy aluminum and measured
against a machinist's square. Note the gap at the bottom between
the part on the left and the square.

Below are more photos of the same two parts. This time, each is placed on its un-machined face and the side is measured against the square. The cut edge on the piece cut with low-taper waterjet cutting measures perpendicular to the reference surface.

llustration : 0.5 inch (12 mm) thick low-taper waterjet part with reference machinists square

Illustration : 0.5 inch (12 mm) thick regular taper waterjet part with reference machinists square

Low-taper and width

In some areas on low-taper cut parts, it's not always possible to completely remove the taper. This can happen if there is an especially small hole or slot in thick material. In these sections, the waterjet head will tilt to compensate for the waterjet stream spreading out, but then the side of the stream opposite the cutting line tends to cut into the material on the far side of the slot.