There is symbolism associated with the finger you wear the signet ring on. In some countries the pinky is considered to be close to family. That is why many heraldry rings are usually worn on the pinky. A signet on the pinky can also be associated with power. Design-wise a ring on your main fingers such as the index finger or the middle finger will give the ring a more prominent position. A subtle design will therefore play better on your main fingers. While a more extravagant design will do well on your pinky or thumb.