Don't be afraid to go down in the octaves. Some vocaloids are better in the lower octaves than higher. However, some users feel less confident about straying too far down the piano scale. As well as octaves, pitching the voice can control the Vocaloid's results (note; don't confuse Octaves and pitch with each other). Big Al, for instance, is one of those Vocaloids who feels more confident low than high. In contrast, Hatsune Miku enjoys higher notes to Big Al; however it is also possible to over pitch her, making her voice squeakier than it normally is. Prima is also another Vocaloid commonly seen being pitched too high.
As part of learning about how to use your Vocaloid, you should be trying to find the voice that works for the Vocaloid best. However, this remains one of the most common mistakes for a user to make regarding a voicebank and is seen regularly within songs, especially cover songs when no adjustments are made to compensate for differences between the Vocaloids. While it is easy to correct, many fail to spot the mistake.