I do one after I clarify (its my rinsewater for the conditioner), and after rinsing relaxer from my hair (but before the shampoo stage).
I think it really depends on the ratio that you mix it to.
When i'm clarifying I mix a gallon jug of water to 2 teaspoons of organic ACV. Last time I relaxed I used a 10:1 ratio of water to ACV.
Anytime I rinse with ACV its using cold water. My hair likes it cold.*shrugs*
When I'm rinsing after clarifying its so that my hair doesn't mat up and get really tangled. It also makes my hair nice and soft too.
After a relaxer its to bring the PH down as well as neutralize the relaxer.
ACV has a PH of around 2 to 3.5 I believe (I guess it depends on what stick you're using)
-A