The correct way is vertical cross and usually with number one blade (marked with a black or yellow circle) at the top, especially if the aircraft is to be parked for more than a few hours or any length of time in cold weather conditions. This is to prevent oil getting past the Beta Feedback Shaft Seals. I am reasonably sure this is mentioned in Safety and Servicing Notes and the Prop AP/TOs and there was if I remember rightly a Lockheed Service News article about it.

The other very good reason for leaving them in + not x is that during your preflight walk around you check that the Prop Brake is working and also the prop and engine are free to rotate. If you leave them in X you have to get a ladder out to do the check. (There was a Flt Engineer known as Micro Eng who had to get a ladder out for the inboards but that is another story).

The Americans would often use the X though and the only explanation I ever got was it made the aeroplane look sexier.

This is the ‘correct’ reason. If the aircraft is to be left longer than overnight, then No.1 blade to the top to stop the oil draining out.
If you’re parking up overnight then you can leave the blades where they’ve stopped.