If you are using FFF/FDM printing, you don't have to hollow the model. A low infill percentage will give the model internal strength without excessive filament use.

If you are using a resin printer, just about every model should be hollow, except for those that are so small that the wall thickness matches the horizontal resolution of the printer. For example, I had a request for a resin printed model which was 2 mm wide by 4 mm long and 1 mm tall. It's not practical to hollow something so tiny, but the part had a hole in the design that allowed for drainage anyway.