My experience (12 years post diagnosis) has been that particular symptoms have started, developed and often stabilized. So, although I feel that my PD has steadily got worse, no single measure captures that decline.
For instance, to start with I had a small right hand tremor, but no left hand tremor. After, about 5 years post diagnosis, my right hand tremor was unchanged, but my left hand had developed an even worse tremor. But, since then the tremor has not changed much.
Another example, is that to begin with I had no drooling. It started after about 8 years, but hasn't got any worse for the last year.