Is it necessary to choose a significator for a reading? No not necessarily, but it can help lend deeper insight and get you to think more about yourself and how you might make better decisions. For me, a significator helps with objectivity. You can look at the characteristics of that card as another person or a friend and ask yourself "what advice would I give this person based on all the cards in this spread"?
How to Choose a Significator
When doing a reading for yourself you should choose the card that you feel represents you now, in the current environment of the question you asked. If you are a business owner for example with a profitable business and you are seeking guidance on finding partners or otherwise making new business moves you might choose to pick a shrewd, powerful man or woman. The Queen or King of Coins are good choices. If you are choosing from the Major Arcana the Priestess, Magician or Emperor may be good choices here.
The key is to choose the card that you feel best represents you at the moment and within the context of the area you are seeking advice on.
Some of us have a card that we resonate with all the time. For me, I often wear a priestess emblem around my neck. I am very drawn to the energy of this card and her characteristics are what I personally strive to be. She is intuitive and deeply feeling, but distanced enough from that to be objective and discerning. She represents the best blend of logic, creativity and intuition. As such, I find her to be well-rounded and capable and I strive to be the same way in all of my endeavors.
She is often the significator I choose in readings, but not always. Other times, depending on the situation I am seeking guidance for, I may feel I resonate more with a different character of the Tarot and that's fine too.
Which Cards Do I Use as Significators?
Typically most people stick with the Major Arcana and court cards as these are the cards that tend to most represent specific personality characteristics. Any card could conceivably be used however so don't feel you have to limit yourself.
The Court Cards all have various personality traits and they are also associated with different Astrological sun signs. I caution against using a “sun sign” to define a person outright however, because I don't find that to be the most accurate. Go with your intuition when choosing a significator for another person, or help them to choose one for themselves. I find choosing one's own to be the most effective, because it means the questioner has a bond or draw to that particular card. What a questioner chooses can also lend insight into your reading.