I've had planted tanks and fw tanks for quite a while now. The reason for removing your filter sock is to minimize using filter media that is replaced as a substrate material for beneficial bacteria. Since your sock will likely quickly clog up with detritus, and need to be washed and eventually replaced, a large portion of your bacterial colony will be lost, at least that's the thought...Sponges are far preferable because when they clog, you can unclog them by squeezing them out during a water change, and putting them back in the filter. Essentially a filter you never have to replace.
IMO, as long as you heavily plant the aquarium from the outset, you don't really need to cycle the tank. Many plants are capable of utilizing ammonia in the raw form. I usually add a light stock into the tank on day one. Also, in my experience, the foam media is far preferable to a sock or other fine filters - it doesn't clog often, adds another surface for beneficial bacteria, and doesn't need to be replaced often. My understanding of filter socks for saltwater is that you want to quickly remove detritus before they have a chance of rotting and spiking nitrates. That's not as big a problem in FW planted tanks, assuming they are planted heavily. If you are using a filter sock, I would also recommend making sure your filter isn't doing too much aeration - you want to keep as much dissolved CO2 in the water stream, and any turbulence effectively scrubs this from the water. This is why almost no one uses a sump, and people prefer canister filters in the FW planted world.
