I wouldn't know the answer to that without looking it up Pavane, although I could perhaps point to an example where we found gracenotes and slurs behave differently with our site (abcm2ps) and concertaina.net/tunatron (abc2ps)
A certainty though is that in the past, abc standards have been ambiguous and open to more than one interpretation.
My personal hope is that when finalised, abc 2.0 will clear up these sort of things. abc has excellent potential, has some excellent developers (which may well include you and Harmony) but IMO is plagued by this "simple" flaw. I can for example be pretty confident that our abc at folkinfo will produce a reasonable graphic or MIDI but I can't have the same confidence over what other abc programs may do with the raw abc.