Typically what I will do, just to be sure of the order of when things occur, is to create another groupheader (add section before) and I will place my reset formula there and suppress the section.  This way, I know that the variable is reset before it goes to a subreport, and if there are no records in the subreport, it probably won't reset the variable as it is not called...or so it seems.

 

HTH