m-angler:

I've tripped with down for at least two decades. I like granite gear compression sacks lined with good garbage bags (steel saks the preference), the garbage bags get folded closed (twist em and you can't effectively compress the bag) and then the bag will compress to the size of a cantaloupe (15 degree bag). The combination of compression and trash bag is very water resistant. I've yet to get my bag wet and I don't use a pack liner.

I've always used a compression bag for my down bag.  I use a waterproof compression bag from Sea-to-Summit, IIRC.  Never had a problem with compacted down.  I just shake the bag out and fluff it up a bit when it comes out of the stuff sack.

Unless you leave it stored packed tight, you have decades of use out of a down bag.  Unless we're talking about only July/August nights, a synth bag has at most a 5-10 year life of it's stated temp rating.

Sleeping bags and anything that lofts for insulation should be stored fully lofted in the ubiquitous cool/dry place.

I pack my sleeping bag with a heavy duty garbage bag liner in the stuff sack.  On a long trip I might use two and stuff a spare or two. 
Once the bag is in the stuffsack I put all my weight on it and beat it about to get the air out.  Then I twist each liner bag and stuff the end down the side to secure.  No knots, knots in garbage bags become rips.
Then the compression straps go on and it's secure enough for me.

My preference for waterproofing everything that needs it is to deal with items individually or in small batches.  A single waterproof pack needs only one failure to get everything wet.  A bunch of waterproof sacks in a pack that's just abrasion resistant is better piece of mind for me.

My pack floats.  The only things in it that can get wet are things made to get wet, tarps, rope, stuff sacks.   One should be able to paddle one's pack home.   

I'm of the opinion that really cranking down on compression sacks isn't good for a sleeping bag.  

You can break a soft steel bar by bending it back and forth enough times.  Kevlar fibers get brittle and break after repeated bending.  Aircraft hulls develop stress cracks after repeated expansion/depansion (ha! I made up that word) cycles.  So why does anyone think that supercompressing down isn't bad for the down fibers?

There is a difference between stuffing the bag into the usually almost too small stuff sack you get with the bag, and doing the compress the bag with cinch straps till it squeals overdoing it.  Your bag will last longer and maintain more loft the less you break the down fibers.  How much is too much, and how long it takes before you lose loft is probably direcltly related to how much you crush the bag.

For the most part, I don't see a whole lot of advantage to super compressing a bag - you gain only marginal space, as compared to the normal stuff sack you get with most bags.  IF it made the difference between taking pack A and a bigger heavier pack B, its probably worth doing, but otherwise why bother (I know, I know, its like crushing aluminum beer cans on your forehead - it just feels good)

If you look out on the many backpacking forums, you see a mix of opinions on this - no way is it decied/proven/concensus that compression sacks won't hurt your bag - eventualy

I have 4 Marmot bags, one mOuntainsmith and one Montbell bag that are all in the 800 to 900 fill range - none of them came with compression sacks - wonder why?
the only two bags I have that came with compression sacks were an REI bag and an EMS bag - both of lesser quality and more dubious temp rating.  Unless something has recently changed, you don't see the best quality bags coming with compression stuff sacks - wonder why

One reason for that I think, is that the main reason for buying a quality down bag is to reduce weight  - it is counter to that idea to bring a much heavier compression stuff sack than it is to bring the normal, lightweight sack that is included.  And since I go as ultralight as possible, I use even lighter stuff sacks that save me 3 or 4 ounces or more per bag.  That is a better tradeoff for me - i.e. less weight vs less volume.

edit: - not sure, but I think the original intent of compression stuff sacks was to make it easier to pack your bag - i.e., you were given a much larger size stuff sack so it was easy to put the bag inot it, and then you used the compression straps to pack it down to a more reasonable size.  nothing wrong with than concept if you don't overdo it.

Android -

YES - change the bags - my experience at end of May first week of June in Quetico is that a 40 degree bag is not warm enough.  A 30 degree bag is generally about right, knowing that i can add a fleece jacket or polypro underwear and wool watchcap.  A twenty degree bag will hardly ever be too warm for that time of year.  I normally bring 30 deg bags now.

I have seen ice in the canoes on several trips in the morning, and frost on the tent one morning is not at all unusual.  I think coldest morning was two years ago, on Memorial Day at Dawson Campground - It was no warmer than twenty degrees that morning.