It depended on the situation because, as they say, "tracers work both ways." You can see where your rounds are going but the baddies can see where they are coming from. In the bush the M-60s had 1 in 4 tracers and some folk used a tracer round near the end of a magazine to signal that there was only a round or two left.
In contrast, when I was in the rear and got sniped at the bad guy knew where I was already and my goal was to make him jump and to disturb his concentration while we hauled ass out of the area.
Besides just the disturbance of having tracers incoming tracers were generally associated with machine guns. If he thought for a moment or two that he was being fired at by an M-60 instead of a Thompson that was all for the good.
I hope this is only of historical interest to you and that you never have to use this information in the real world.
late of 1st Plt., C Co., 1st Bn, 12th Cav., 1st Cav Div (airmobile), RVN, 1970-71