I don't think it upsets the energy of the deck to look for one, but I don't use a significator. Raymond Buckland says if you want to use something for a significator, it needn't be a card. It can be a coin or something else that belongs to the querent instead because that will have their energy associated with it. Some people have the querent write their question on a slip of paper and put it under the reading cloth or somewhere like that.
Also, when first starting out, the Celtic cross is really too complicated. I think a smaller spread is much easier to use and can give plenty of information. Many people use just three cards to begin with and the meanings of the positions can vary, as in body, mind, spirit, or problem, possible solution, outcome, or past, present, and future. I know lots of experienced readers who seldom use something as complicated as the Celtic cross. For myself, I use seven cards---the first two for the past, the middle three for the present, and the last two for the near future. Sometimes I vary the meanings of my positions, but seven cards seems to be my most workable spread no matter what meanings I associate with the positions.