Why are 4 blade propellers often in X or + position when the aircraft is parked?
I have operated twin engine aircraft with four bladed propellers. While not warbirds, the following should illustrate some of the reasoning that goes into positioning aircraft propellers. Each type and situation could vary.

In our operation we have two positions in which we place the blades when the aircraft is parked. The positions are the x and + position you mention, and we will choose the most appropriate one dependent on the situation.

The + Position

When we park such an aircraft on a ramp we may want to secure the propellers so that they cannot turn in the wind. One common method of securing the propellers involves placing the propeller in the + position and securing the bottom blade with a cuff that is tied off to the sides with a bungee. 

These propellers are bolted to PT6 series engines in which the propeller section (power turbine and gear box) is free-turning. Securing the prop helps ensure that the gear boxes do not turn without proper lubrication, and helps reduce wear.

The x Position

When the aircraft is parked in a hangar or on a ramp with the propellers unsecured, which is more common for our operation, we want to place the propeller blades in the x position. 

This is to put the blades in a position which poses the least risk to people moving about the aircraft. The height at which the blades would sit in the + position would typically put the blade tip right at head level, a dangerous position for a relatively sharp and low visibility aircraft part. Conversely, placing the blades in the x position places the upper blade tips out of the way and the lower blade tips down near knee level.