Gavon-
that would depend on whether it is progressed transit or natal , changing direction or house placement or whether it is intercepted or not all is determined whether it is poorly placed by house or sign. If it is going direct or it has just went RX which planet it is and so on . There is no easy or single answer to the question unless it is more specific
My question to your comment on "look at the degrees when merc is going retro. & u may percieve more "
Is the perception more keen or is it that you have gone back to it and uncovered more insight to what you first glanced at..?
I don't see Rx as a way of perceiving more but rather I find the stationary time is when the perception kicks in. An additional slightly off track but on the the topc tidbit, in an event chart, in some cases , irx can bring about a good time for resolving loose ends