What airport are you flying out of? How big and how busy is it? What day of the week are you flying? Are you checking luggage?
I usually do stick to the 2 hour pre-arrival time, as I would much prefer to be early over the stress about being late. Early mornings can be very busy at urban airports as lots of business travelers are flying first thing in the morning. You want plenty of time to get through security.
That said, are you able to check in and print your boarding pass on line? That definitely cuts down on wait time at the airport. Also you need to check and see what time are ticket counters and security lines are staffed at your airport.
Once you think about some of these issues you will better be able to judge if 90 minutes is enough lead time.
Good Luck.