I would usually use V/S to start the descent and maintain the descent Mach No until intercepting 300kts and hold 300kts in VS
for a bit until it was stable and then engage IAS hold.
Mach hold on descent wasn't very good it tended to chase the Mach No once it was disturbed not very comfortable, the same thing could happen in IAS hold.
ie if accelerating through say 280 kts towards 300 in VS
and you selected IAS hold as you got to 300 the A/C would continue to accelerate to say 310-320 pitch up to get 300 back but then go through it to 290-280 pitch down again and so forth. I loved the fact that the Autopilot wasn't perfect and it took a little skill to make things happen nicely.
I tried several different options to see what worked and what didn't.
Once descending at 300kts in IAS hold and nearing 10,000FT I would select VS
reduce to 500FPM and slow to 250kts then increase the VS
to hold 250kts for a bit then reselect IAS hold.
Ideally an idle descent would be made and VS
just to reduce the descent rate when low on profile. IAS hold and advancing thrust to reduce the decent works but was never as tidy as VS
and using the thrust to control speed.
However there is more than one way to skin a cat and I'm sure there are plenty of other versions.
P1 in A330, A340, A346, B742, B744, B748.