- Don't use tricks, which you don't understand fully
- Don't use tricks, when they aren't needed (subtree merges is a must in a rare, some special cases only)
- Read relevant part ("Subtree Merges and Subtree Mergeinfo" topic) from SVN Book gently, slowly, without haste
And now, considerations regarding your question:
- When you merge some revisions in subtree, in means nothing for parents - because for parents of any level these revisions are not merged (even if they haven't any objects outside subtree)
- Mergeinfo of subtree used only for avoiding duplicated merge in this subtree (not above or below), when the same revision(s) merged later to some parent (and moved to merge-root of parent, when such merges with intersected revision (in subtree and parent merges) is done)