A couple years back I did a light duty survey of people using down bags. It turns out that for most compresions of 10:1 or less there is no damage to the down or lofting. This was for high fill power downs, not for lower fill power downs. 700FP and above was my cut off(by EN measurements.) It doesn't really matter if you use a tight compression.
Dampness, wet, or real compression (say with a hard object pressing down on it) can damage it to some degree. But generally down is very forgiving. Shaking it out good is generally all that is needed. A blast of warm air in a dryer when you get back will restore it if it does get damp. Warping of the fibers due to water is NOT permanent. But, be carefull not to damage the shell with heat.
I usually use a compression bag these days to save packing volume. It is compressed till it *just* fits into the bottom of my bag (also my down camp jacket and sleeping socks.) Roughly, this is about an 8:1 compression. The eVent dry bag also keeps things pretty dry. Other than 2 day rainstorms, it never even gets damp.
But, the compression/dry bags weigh about 2.5-4.0oz or so. Heavy for just water protection. Worth it on a canoe trip through the ADK's. You are better off with a pack liner (compactor or turkey roaster bag) for just hiking, though. Mike C.'s book had a humerous description, LOL. Lots'a laughs in that book...