I have a valid ESTA. When I enter US I will show my return flight booking to leave US. Should I decide to stay longer while I am there, but still within 90 day period, will my rebooked flight date be acceptable?
When you arrive at a port of entry in the U.S., the Customs and Border Protection officer decides how long your visit is allowed to be, and this information will be stamped into your passport as part of your entry stamp.
Most of the time, this is 90 days, but it is possible that, depending on your individual circumstances, you will be allowed less than 90 days on a visit. Regardless, you must leave the US on or before the date stamped in your passport.
I would recommend that when you arrive you declare the maximum amount of time you might want to spend in the US. There's no penalty for leaving early, but the penalties for leaving late can be severe.