Yes I believe they did. Here is an example from Vietnam on how 50 cal M2s were used on trucks and how they were loaded. I don't think this was anything newly invented for Vietnam.
http://books.google.com/books?id=6Ae...tracer&f=false
They set up two types of 100 round belts. One used 2 incendiary, 2 AP incendiary, and one AP-Incendiary-tracer. The other type of belt used 4 ball and 1 tracer.
In Gordon Rottman's book on Browning 50 Caliber Machine Guns, on page 37 he mentions the standard mix for 50 cal used in an antiaircraft role as:
"At this time (early WWII) the functional ammunition mix was four ball to one tracer, two armor-piercing, two incendiary, and one tracer, which was excellent for antiaircraft use."