Kinda depends on the sleeping bag. A synthetic fill sleeping bag won't recover from squashing as easily as high-quality down!

I haven't noticed any difference in my sleeping bag (850 down) from its being squashed in my pack during the day, but I did notice a difference when I squashed it into my pack and then drove two days before the trip!

I think a lot of us are a bit paranoid because of the $$$ invested in that Western Mountaineering (or comparable) sleeping bag!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey