I always fly with the txpdr on Mode C. Seems stupid not to if you have it....after all its my ass up there !
I was a bit surprised when first flying in the UK, coming back into a class D zone in the south of England, the instructor I was doing the check out with told me to turn off the txpdr as the ATC "didn't like all the 7000s all over the screen!"....seemed a bit alien to me, especially after flying around the LA basin where air traffic is about 100 times what it is in the UK.....