It's no surprise (ok, sometimes it is if you don't read the small print) that some airlines charge for seat reservations on top of the ticket price.
Generally we are not too picky about seats as long as they are next to each other. We recently traveled from Portugal to San Francisco via Lufthansa and did not pay extra for seat reservation. Upon check in we were assigned seats (together!) so it all worked out and we saved around $60USD. Sure prior peace of mind that the seats would be placed together would be nice, but is it worth the extra cost?
From my traveling experience, the customer service reps generally try to sit a pair together whenever possible. This is probably because, one person will most likely have to sit in the dreaded middle seat. If you're flying solo, I'd pay the extra money to get a seat I really want, especially on a long hall. On a short hour, or 90 minute flight, i just deal with it.
The best tip I can give you is to call the airline. If your traveling on a mainline airline (Air Canada, Lufthansa, Qantas, etc...) most of the time if you call the airline asking for seat changes they'll give it to you for free. However make sure you know which seat you want and if it's available before you call it makes the process easier. You can find out if it's available by going to your reservation online.
You can also book via a travel agent most of time they can seat you together without a fee.
If your traveling on budget airlines however your chances of getting pre-book seats are less likely. However if your flight is not full you can always change seats as long as it's the same class.
There's 2 versions of this money grab.
Version #1: Airline wants money just to be able to get an assignable seat earlier than 24 hours before the flight. Guilty party is British Airways.
Version #2: Airline holds nicer seats from seat assignments unless you pay extra for them.
I can tolerate Version #2. But I avoid it. So I simply select my airlines better. Paying for more legroom is fair if you need it.
I don't like Version #1 because I think it is a scam. So, I do not fly BA.
All the airline charge for the same, even if you choose seat near exit gate then they will charge more, I have faced the same situation in India when I was traveling from Jaipur to Kolkata via Indigo.
I always select my seat in advance, even when I end up paying for it - for longhaul flights. I have experience with a 12 hour flight where I ended up in a middle seat and that was not convenient at all.
Almost every airline now charges you to make a seat choice, unless you have frequent flyer status with them. For several years I paid extra on Qantas to book exit row seating in economy class when flying long haul. The plus is extra leg room but the minus is that a lot of these seats are next to the toilets and people tend to stand, wait, congregate and chat there so not a particularly relaxing place to sit. I've now stopped doing that. The cost, particularly when flying from Sydney to NYC was also quite high.
Basically, I look at what you get for your seat reservation. Last year on Hawaiian airlines, I chose to pay a little extra for specific seats for my flight from San Fran to Honolulu. I think it was $15 more per person and I got a great seat in a quieter, smaller part of the cabin and a fairly decent meal. It was certainly much more relaxing than either United or American (having done that trip with both) and even though my husband and I travel together (on the same booking) we were not seated together.