notehead offsets in single-staff polyphony

[Mike Blackstock]

Guitar music has a lot of this stuff:

\relative c' {
 <<
   {g'16 d' b d}
   \\
   {g,4}
 >>
}

where the G in the top part lines up with the G in the bottom and they 
share the same notehead;  exactly as expected and wanted.

However in this:

\relative c' {
\time 3/8
 <<
   {g'8 b d}
   \\
   {g,4.}
 >>
}

the dotted-quarter in the bottom is offset to the right, 
counter-intuitively (to me at least)

In the past I've worked around it with this:

\relative c' {
\time 3/8
 <<
   {g'8 b d}
   \\
\override NoteHead #'X-offset = #-1.6                      
\override Stem #'X-offset = #-1.6
   {g,4.}
 >>
}

but then i have to keep switching back with "\revert NoteHead #'X-offset 
\revert Stem #'X-offset" plus I'm just eyeballing and guessing at a good 
offset value .

This can't be the right approach. Is there something in the manual I've 
missed?  And how come the different behavior in the 2 examples?

Thanks. - PS the quickest way to visualize the above is at  
http://draft.wikilily.org/wiki/index.php/User:Mike_Blackstock#Sandbox
-Mike