I forgot to mention a really important reason why we like September trips in Quetico.  It's something to look forward to for the entire spring and summer!

I am a late August guy as well, less crowds/bugs. I would like to do a September trip once my schedule allows it.

Have always gone in very late August or in September.  For the past 5 years it's only been mid-September for the reasons stated by others.

Mid-September can be warm and beautiful or cold and windy (or sleety even as it was last year) but there are very few people. You never have to rush to get to a particular site (other than for weather-related reasons) and the bugs are virtually non-existent.

The days are shorter but the stars at night are brilliant and that first sip of brewed coffee on a cool, crisp September morning is exquisite.

Last time I was there was a beautiful September. Not a lot of others. That's my favorite time of year in just about any place north of the equator.

My favorite time to be in the Q is the last two weeks of August into the first week of September. Next to no bugs is great, especially on the portages and at night by the fire. The cosmos puts on a pretty good show with meteors, the fishing is pretty good and I generally don't see many people. I time my trips so that if I'm in the park during Labor Day, I'm at least two days from an entry point, which really helps with seeing few people. Last year I got to see the colors start to change which was pretty cool.

I personally tend to perfer around the 3rd week in June. Love the cool nights, when campfires are graet to snuggle around. The cool drinking water is always refreshing. I fish a lot and have cut back on travel somewhat, so this period is for me...fishing is usually ridiculous for all species. Much easier to find the 5 star campsights open also. Last few years, bugs have been surprisingly easy to deal with for some reason.

Agree with you all however, that any day in Quetico is better than the best day at work!

Spring.  Wherever I go, I love spring the most.

The world is fresh again. 
The smell of damp loam.
The sound of water burbling down portages, cleaning the muck away & leaving hard ground to walk on.
The water is clear and clean and cold.
Campfires have purpose.  Firewood is plentifull.  Fire bans are exceedingly rare.
Trout are up top & hungry.
Crowds of fairweather campers have not arrived and left their messes behind.
A nice day is a beautiful day, all things relative to expectations.
Cooler on travel days.
A crappy day is another story to tell.  Who tells stories about the trip when everything was perfect?  Let me tell you about the time nothing happened...  Most stories I hear are about the hardships, the rain, the wind, the snow, the storms, the bragging rights stories.
Animals are out and hungry, though I often see more moose & bear on the highway.  More sightings.
Higher water levels, every step taken from the portage is +1 for me!
No bugs.

2nd fave is autumn for mostly the same reasons or their corollaries.

Summer is just too darn hot.  I do like swimming on a hot day, I'd just prefer the temp to be 5 - 10C cooler.
Winter needs $2k in additional equipment to be reasonable.  Hot tent, pulk, snowshoes, clothing.

I love long days and good fishing so lately I've been making my trips on the 10 days prior to July 4th.  I always plan to come out on the 3rd so I can enjoy the holiday back home with the family without cutting into my tripping time.  Plus it gives me an extra day to get out if the weather turns on me and I can't make my planned exit on the 3rd.

I find the summer peak fishing period generally happens right around this time of year.  If you hit it right on, the walleye are strapping on the feedbag, the bass have moved off the nests and the females are ready to rock again, the lakers are still catchable, and big pike are on the prowl.  If spring comes late, you still hit bass on the nests, at least some walleye are catchable, big pike are still relatively shallow, and lakers are easy targets.  If spring comes early like this year, you still get good fishing and the blueberries are starting to ripen early enough to scrounge a cup or so for the pancakes.

Bugs are also starting to mellow out.  I generally miss the worst of the blackflies and the skeets are manageable during the day.  Daylight is still essentially at it's max, too.  Matter of fact, it's almost impossible to get enough sleep and still use all the available light.  Not that I really worry about sleep though.  I can sleep at home.  I want to spend as much of my time awake as I possibly can.

Have you checked out the recent post on Ice Out???

Any time I can get there, is the best time to go.