For those of us who don't fly enough to get preferred status, sitting towards the front on a very full flight could result additional delays deplaning because:
1. Without preferred status, seating order is usually back to front
2. By the time the last "group" is called for boarding, many of the overhead bins are full
3. Full overhead bins means that you have to check carry-on luggage at the gate (a hassle)
4. When deplaning, you usually have to wait in the jetway for your bag while everyone else who sat behind you walks by.
5. There is a chance your bag will be misdirected either to baggage claim or elsewhere.
1-4 has happened to me a number of times...fortunately 5 hasn't; although, I have seen folks waiting when the baggage handlers said that's all they have.
Now, I shoot for a seat in the middle, unless I'm pretty sure the flight is not very full. I am usually just short of the preferred status each year as I don't fly all that often and my flights are mostly in the same region.
P.S. A nice addition to seatguru.com would be typical "zones" that the Airline uses for boarding order.