As flight crew we time as I think its fair to say, most of us have been subjected to the occasional mistime on wake. I've personally been cleared for takeoff immediately following an A330 when in an A320... Needless to say the controller was very humble when he noticed the error. Then asked if we had a clock running...
Anyhow, I think the issue in this thread is interesting, in that we time differing things. I almost always see colleagues time the start of the T/O roll whereas I time the rotation ever since an ex atc colleague mentioned the difference in techniques. I simply feel the more people who are aware of the differing timing techniques (and the reasons) the better it would be...