Dear Chik-Sum Gong:
If you really want to purchase the property you should offer the full price. If you do not want to offer the full price you can offer anything you like, just bear in mind that the farther away from the price you are, the less likely it is that you will make a deal.
When looking at houses there is no general rule of thumb (i.e. always offer 10% less). You have to look at each property on a case by case basis, look at the price tag on the property and ask yourself, is this a good deal? Based on your answer to that question, you can make your offer accordingly.
If you want to discuss this further and/or want more specific input, feel free to contact me. Good luck!
Mitchell S. Feldman
Associate Broker/ Director of Sales
Madison Estates & Properties, Inc.
Office: (718) 645-1665/ Cell: (917) 805-0783