I found that leaving my bag compressed in the pack for several days while driving home from Wyoming (I didn't unpack it until the day after I arrived home, either) reduced its loft a bit, although it recovered after washing. I'm pretty sure the damage was due to the bag's being slightly damp from the last night's use. Now, unless I'm close to home, I take the sleeping bag out of my pack as soon as I get back to the trailhead and put it in its storage bag. Ditto on the way to the trailhead if it's a multi-day drive; I don't pack the sleeping bag until the night before I start backpacking.

I store my insulating clothing uncompressed for the same reason.

Unless I'm going out for a really long trip (like 8-10 days), my backpack isn't heavy enough to justify using it for conditioning. 10-12 lbs. in my daypack seems to do the job just fine, and the latter is equivalent to my backpack base weight. I've also found that carrying extra weight when walking on sidewalks can do nasty things to my feet! I now use the daypack only when walking on trails or on the path in a local park.

Just one person's practice here; YMMV!