Although the other agents have good points, I disagree. I think it is good for the seller if the listing price is close to assessment because it presents value to a buyer- it helps the price of the home seem "not inflated" if they feel they are buying the true value of a home. Assessment is the value of the land plus the value of the structure - by the town.
A year ago, in a different market, assessments were a very different number than market value, however in this market, buyers are looking for a price closer to or even under assesment. For this reason, I think assesment is now playing more of a part in the market.
If the house sells below assessmant, you can ask the town to re-assess the taxes, but there is no guarantee.
I hope this helps. Good luck.