Whenever I look at a seat map, I always see middle seats selected when nearby windows and aisles are open, particularly within a few days of departure. Of course, if a flight is completely sold out, most of the seats are selected in advance, but it's clear that many passengers are accepting their randomly assigned seats and not making the decision on their own after completing the ticketing process. There's absolutely no advantage to not selecting a seat, unless all of the seats in the basic Coach Class section are taken -- in this case, there's a possibility that you will be assigned a seat at the gate with additional legroom, though if that does happen, it'll probably be a seat in the middle.
Be careful though: if a flight is overbooked and you don't have an assigned seat, you could end up getting bumped.
Ideally, after you finish making a flight booking (or during the process, depending on the airline), you should click to view the seat map and make your pick. If you book your flight with friends or family members on the same reservation, they will likely be assigned seats nearby automatically, but there may be better alternatives available, with multiple open seats in the same row. If you have a few minutes to spare, check out your aircraft's layout on Seatguru's website. Good, not good and poor seats are clearly labeled using color green, yellow and red squares, respectively.
It's a bit of a hassle, but it's worth the effort, particularly on long-haul flights.
After you complete the booking process, head over to Seatguru.com and locate your aircraft. Your airline may have multiple versions of the same airplane type, so make sure the airline's seat map matches what you see on Seatguru. If they don't match up, simply select a different version of that same aircraft. United Airlines, for example, operates six different versions of its wide-body 777-200. Some of these have updated cabins, while others are more dated. There are also two different types of Business Class seats on the international configured planes, so pay very close attention when you go to match these up.
If you haven't already guessed, green seats are what you're after when viewing the map on Seatguru. In the Coach cabin, these are typically located in rows that require an up-charge. Some airlines call this "Economy Plus," "Main Cabin Select" or "Even More Room," just to name a few. Regardless of the name, you can expect to pay anywhere from $30 to $130 to select a seat in this section, depending on the seat and the length of the flight. After that, seats that don't have any color coding are fine picks, too -- these won't have tons of extra legroom, but they're average seats for that cabin.
Generally, you'll want to avoid yellow and red seats, as these often come along with a negative bullet point or two, be it a position near the bathroom or galley.