Straighter the better, but only to a point. As you said, they do need a little angle to cause some movement for even wear and lubrication.
In the "Installation Techniques" manual (second of the two links above):
Rule #1, Universal joint operating angles at each end of a driveshaft should always be at least 1 degree.
Rule #2, Universal joint operating angles on each end of a driveshaft should always be equal within 1 degree of each other (1/2 degree for motor homes and shafts in front of transfer case or auxiliary device).
Rule #3, For virtual vibration free performance, universal joint operation angles should not bee larger than 3 degrees. If they are make sure they don't exceed the maximum recommended angles.
Grigg
