Shift-left-click does not take you into drawing mode for fields with the "cuboid: true" flag in Precious Stones 8.3. I have tested this with:

CraftBukkit 1.2.3-R0.1
Precious Stones v8.3
PermissionsEx v1.19 (giving the default user the '*' permission.)

With PS's default config.yml on a fresh server, I place down a diamond block (City Protection) and try to resize it with shift-left-click. Nothing happens and there is no console crash or other error message. Debugging with "/ps toggle debug" shows that no precious stones permissions are being checked when I shift-left-click.

Tested and it works without problems. Make sure you are holding one of the tools configured in the "tool-items" list in your hand.