Hi. Every audio system is different, so there's no "one size fits all" solution. I believe that audio and video components are most often best served by being isolated from the shelf/rack. The shelf/rack (as well as floor and airborne vibrations) will have some influence no matter what, but effectively isolating the component from the shelf and rack reduces the sonic influence of those parts.
With most audio and video components, I believe, you'll get a better sonic result holding the component in the palms of your hands (isolation) than you would clamping it in a vice (coupling).
The more sonically neutral a rack is, the more effective isolating components from the rack will be. The main problem with isolation has been that many isolation materials themselves are more sonically intrusive than the rack itself--in such cases, coupling to the rack would make sense.
Rigidly coupling a component system often tends to deliver a harsh, rigid musical result, though. Micro-vibrations don't always follow the "rules" of draining and behave more like ants at a picnic. With audio components, it's usually higher-frequency vibrational energies that you cannot feel that cause much of the glare and grunge that we're trying to get rid of. These acute, angular vibrations tend to travel readily through rigid materials (which virtually always add some sonic character of their own). As an interface between component and shelf, compliant materials that smooth out, block, and absorb these vibrations will deliver exceptional sonic results as long as the materials themselves are basically sonically neutral.
In the past (and presently), the problem with isolation has been the materials used produce some resonances and reverberance on their own, so whatever benefit they achieve is at the cost of "trade-off." Herbie's Audio Lab does not believe in "trade-offs." As you improve your system, each incremental step should bring about improvements only, not improvement in one area at the cost of another. Otherwise, further down the road, by building your system around a weak foundation, you'll never achieve the full potential that your gear is capable of.
Loudspeakers are a somewhat different matter, though. Holding the loudspeaker baffle as motionless as possible is the goal, rather than just smoothing out and diminishing acute vibrations. With loudspeakers on a solid concrete foundation, rigid coupling achieves this goal to a degree, though with some reverberation from the concrete going back up through the spikes, potential glare and harshness, and potential floorborne-vibration problems. You wouldn't necessarily want to couple to a concrete floor any more than to a suspended wood floor. The semi-compliant, semi-rigid hybrid dBNeutralizer-based products like Herbie's Decoupling Gliders and Fat Dots are usually ideal. Sort of a half-way between coupling and decoupling/isolation.
This is, of course, not a definitive answer to your question, just my two-cents' worth. You can find more information here: Isolation Feet FAQ's
Steve HerbelinHerbie's Audio Lab