...and isn't that a wishy-washy answer! But at least in my experience, that's been the case.
If you're flying on a busy day of the week (usually Friday or Saturday going down, Saturday or Sunday on the return..or, of course, holiday periods around 4th of July), booking early is about your only choice. The later you go, the more you'll be forced into very early or very late non-stop flights, or non-direct flights with long layovers at out-of-the-way airports.
You'll also get squeezed out of the best fare categories, since the airlines only allot a certain number of seats to those fares. If you're flying in peak demand times, those seats will go fast.
The third factor to think about is fuel prices, which have been rising very quickly. My thought is that rising oil prices will lead many folks to fly instead of driving this summer, which will also increase demand for seats. And if fuel prices rise, the airlines are going to probably have to raise their fares to get that money back -- meaning the longer you wait, the more you might pay.
If, however, you're flying midweek and not returning on a weekend, you might do well to wait. Or, if you're flexible and don't mind layovers, you might even want to consider rolling the dice and trying Priceline, which can get you some great fares if you don't mind how you reach your destination or when you arrive.
Now I'm speaking as someone who flies out of Philadelphia...Chicago might be different, and I hope you hear from somebody who has first-hand experience booking Chicago flights.
Good luck!
Bruce