I do 100% travel and the answer is not a simple yes or no. But typically yes they will cost more the closer you get. Although, sometimes when they feel they are undersold, they will lower the prices in hopes of filling the flight. The best thing you can do is use this site: http://www.farecompare.com/
You can check specific dates and you can also see what is typical costs and what cost would be a good deal. You can also set up alerts and the website will email when the specific flight you want to watch is at a very cheap price compared to average. I love the site and use it all the time. You can get a great idea what it should cost and how the prices change based on how far ahead of time you book.
By the way, 2-5 months is not good adivce. If you want a good price and don't want to run the chance of loosing out then you should book as soon as possible. This would typically be around 320-330 days ahead. I booked a flight from NY to LA for $288 round trip for the day after Christmas about 7 months in advance. By the time it was 5 months away the flight costs $580. Just an example...the will vary.
Shoot me an email if you need help with something specific...hope that helps.
Mechanical and Aerospace Field Engineer with 100% travel.