Obviously, when we are out on the trail, the reason we bring these types of insulation is their ability to compress and fit in our pack and so compressing them is a necessity. However, the loft we use to keep us warm is created by the fibers acting like little spring, holding up the loft. If a spring is left compressed, it loses some of it’s ability to do this. Different materials react differently to this. Synthetics typically will suffer the effects of compression much more than down, which is very resilient to compression. However, it too is like a bunch of little springs that inevitably will react to compression cycles and long term compression. You’ll hear tales of “I leave my down bags smooshed in compression sacks for years. Pull them out and they’re fine!”. Well, they may seem fine, but it’s likely due to the overstuff making up for some loft loss. Maybe it’ll be noticeable, maybe not…but you can be sure that every compression is some level of a dent in it’s ability to loft. So, it really makes the most sense to only compress your insulated items as small as you need, for as long as you need. If you have room in your pack, no need to smash your sleeping bag into a tiny compression sack. Let it fill the space and give your pack structure too. This is not just a consideration for long term loft life, but it is also a factor in trail use. Especially in the case of down. Down is very resilient to compression so long term loft life is much less of a concern than it is with synthetics. However, on the trail, the less you compress your down items, the quicker they will be warm and useful, with less attention required to distribute the fill before use. Obviously, one of the main reasons we use down is its ability to compress and loft back up. So I wouldn’t interpret this as a warning against compressing your down items into the space you have available, but a warning to not compress any more than you need to. There will be benefits to less compression.
All items that contain down (or any other loose fill) should get a bit of attention to distribute the fill evenly, pretty much before every use, especially after compression. When you set up camp, take these items out and tend to them. Move the down around in the chambers a bit by patting, slapping, and pushing. Patting and slapping forces bursts of air through the chambers which will break up clumps of fill that have stuck together. Most of the time, clumps are just due to the fill being squished into corners or narrow parts of the chambers. If you find clumps, it is a good idea to slap back and forth until they break up. Then you can use a pushing motion to get even distribution. After distributing the fill, then let them sit to fully loft up. If you have a horizontal chamber sleep system it usually makes sense to grab the underside of the system and shake the fill towards the center of the chambers, which will be the top of the system when you are in it.