[Jet II: You don't ask for permission to start any hydraulic pump - as for how many systems are presurised that is variable depending on type..]
is un-true...when an aircraft is parked up with engines shut down, the hydraulic sytem(s) will be unpressurised. un-less of course the system is being pressurised by the electric driven pumps, which are normally there for back up.
Agreed, it's extremely dangerour to pressurise the hydraulics on the ground without positive clearance from the ground crew. As mentioned above, you can get people killed by doing it.