Ouch.
These guys are correct, some units did and some didn't use tracers. Francis Gabreski for example is one who only loaded the last 50 rounds in his M2's as tracer . His reasoning was he didnt want to alert his enemy that he was being fired on, and like others have stated, he loaded the last 50 rounds tracer so he would know the guns were low on ammo.
Reference: Gabby, by Francis Gabreski, and Carl Moelsworth.