I think, atleast for the mcat, you only consider waves of the same frequency as "in phase" or "out of phase"
In such a case, a phase difference of any multiple of 2pi or (aka 360 degrees - including 0) is in phase while those with a difference of any multiple of pi (or 180 degrees) are out of phase.
But waves certainly dont ONLY destructively interfere out of phase. Waves destructively interfere when there values are of opposite signs at a certain point. So if you were to superimpose the two waves on a graph, then at any value in which one wave was in the positive y axis and the other was in the negative y axis you'd have destructive interference. This applies to any set of waves, same frequency or not.
hope this helps. btw, are you comfortable with vasodilation/contraction and hydrodynamics? If so, check out my thread and help me out plz! Thanks!