I recently was struggling to deal with this issue as well. The first wireless keyboard/mouse combo I had that I tried to use with a KVM switch was a Logitech MX 3200 Cordless Desktop Laser. It worked perfectly fine with an IOGEAR 2 port MiniView Micro USB PLUS KVM switch (Model No. GCS632U). I simply plugged the receiver into the USB keyboard port on the switch and everything worked perfectly. Logitech's SetPoint software also worked flawlessly with this setup.
After my positive experience with IOGEAR's 2 port KVM switch, I upgraded to their Extreme MiniView 4 Port KVMP switch (Model No. GCS1734). Once again, I plugged the Logitech MX 3200 receiver into the keyboard port of the switch. The results were less than optimal. While the switch was able to detect it from the frontside keyboard port, it took a good 10 seconds to switch between computers as the keyboard and mouse were treated as separate USB devices that had to be disconnected and reconnected upon port switching. This was also true if I connected the receiver to either of the 2 secondary USB ports on the backside of the switch. The only advantage to plugging the receiver into the frontside keyboard port was that it allowed the switch to recognize keyboard port-switching commands.
Later on, my MX 3200 failed and Logitech replaced it for me free (it was still under warranty but had been discontinued). They sent me a Logitech Wireless Desktop MK710. The key part of this is the new Unifying receiver that Logitech features. When I plugged this into the frontside keyboard port of the IOGEAR 4 port KVMP switch, the Logitech MK710 was not recognized at all. Plugging the unifying receiver into the mouse port, that didn't help either. The only way for the MK710 to be detected was if I plugged it into the back of the switch in one of the secondary USB ports. This again subjected me to the hassle of a 10-15 second delay when switching ports as the keyboard and mouse were disconnected and reconnected.
I called up IOGEAR for assistance and explained the troubleshooting measures I had taken and they simply told me it meant their 4 port KVMP switch was incompatible with the Logitech MK710. Total bummer.
So now I'm stuck using a wired keyboard and mouse with that 4 port switch. If anyone knows of a good 4 port KVM switch that works with Logitech's MK710 and/or their unifying receiver, let me know