I think it is good practice but personally, I found it a bit of a hassle and had difficulty in seeing the arrows in the lighting conditions that I was working in anyway.
As Carl says, setting TDC it is about reducing tension on the piston spring of the HPFP which is pressed against the cam lobe. Whenever folks talk about TDC, there is some sensitivity because we know how important it is to get this right when adjusting timing. However, this is a much simpler science than timing....
The lobe is a triangular shaped thing with rounded corners if you get my drift. So when a peak of this lobe is pointing at the piston, the spring is compressed. However, if you can imagine the picture, you would be unlucky if the engine stopped with the piston on a 'peak' simply through probability (even a 20 -30 degree rotation from this point in either direction would mean an exponential drop-off in spring tension). Even if you were a bit unlucky, my personal opinion is that it is not so important when taking the HPFP off as long as you undo the three retaining bolts together and a little bit at a time.
OK, putting the pump back on may be a bit more important but by this time you can look through the hole and, with a light to assist if needed, see the cam lobe and see if you are on a peak or a trough. If a peak, simply rock the car in gear (as you would do to find TDC anyway) until the lobe moves to a trough point and stick the HPFP back on (again I would recommend tightening the three bolts a little bit each at a time).
On saying that, I have done this three times now and have not experienced the engine coming to a halt at a high point / peak in the lobe cycle yet.
