Yes, it is. Infill density is the proportion of filling material for the print. It is an essential feature for strength, printing quality and the economy of filament and time.

Strength: If the design is for carrying heavy pieces of stuff infill rate should be high, at least more than 50%. Of course, it depends on the feature of the design. However, not only the infill rate but also the infill pattern and design affect the print too. If the concept is for light objects, you may decrease the ratio.

Print Quality: High infill rate is more precise most for non-symmetrical object-like character printing. (A dog or a video game character)

Economy and the time: Let's take a look at the phone case elephant above. Our purpose is to give support to the mobile phone. Moreover, we don't need a powerful supporter. Therefore, we can use the same sized object with a low infill ratio. Compared to 0 infill rate, using the infill rate decreases the consumption of the time and the filament three or four times more. Thus, it is possible to decrease the rate up to 10-20% safely.