I have 105 on my bike, double up front with a 10 speed in the rear. When I bought a new wheelset, it had a 9 speed cassette on it - still 105. I just tossed them on and rode a little, but had issues. The LBS told me my shifters were built for 10 speed, so I would either have to change them or get a 10 speed cassette. Since I use my original wheelset with different tires, I got another cassette.
By the way, my front shifters are set to accommodate a triple even though I have a double, so I'm familiar with that ghost shift. It just didn't work on the rear.