I don't think it would be safe to assume a clear rule either way, each ancestor to their own. In my own tree I haven't always been able to identify where a surname used as a forename has come from (and it's by no means only the oldest child that has the 'family' middle name):
Silvester - mother's maiden name (oldest child, daughter)
Wheatley - mother's maiden name (fourth child, second son)
Lewis - no idea, but long before it was fashionable as a forename (second son, and carried down several generations)
Watson - maternal grandmother's maiden name (eighth child)
Pitman - mother's maiden name (oldest child, son)
Powell - no idea (third child, second son)
Pennyman - no idea (fifth child)
Corpes - maternal grandmother's maiden name (fifth child)
Dobson - paternal grandmother's maiden name (seventh child)
Ellis - mother's maiden name (sixth child, third daughter)
Lodge - no idea (fourth child, first daughter).
Wiley - no idea (sixth child)
Warrington - no idea (fourth child, second son)
For all those where I've no idea I've ruled out the maternal maiden name and in most cases the grandmother's maidens names too. All I can say is that some of my lines seem to have a tradition of including a surname and some definitely didn't .
Ermy