Priced too low, you give away part of your valuable equity. Priced too high, the house will not sell as soon or for as much as it would if priced right in the beginning. Regardless of the contract price negotiated with the buyer, the house must appraise or the lender will not give the buyer a loan. Without the loan, there is no sale. Certified appraisers are trained professionals and are not influenced by a seller's need for money or emotional attachment to the property.
Appraisers use several methods to determine value.
If, on the other hand, your home is larger than the comparables, you may overprice your home and end up selling, after a long wait, for less than you could have gotten had it been priced right in the beginning. Ask if they use the average square footage method.