Hey, you see, different/mix answers from everyone...

Now...the question:
"""If you've repaired one leaky/damaged head and left the
other alone on a WBX""""

Big difference between DAMAGED and Leaky..
Leaky = why not... if you're on a budget or a rush (even then!!!!)

Damaged = here, you need to be more precise... if an head is very old and as
a crack or droped seat or any other severe defect....you need to understand
what cause that problem (if possible). And that probleme may afect the other
side sooner than you think.... and we all know that old VW reman heads are
not the best....

Another thing, on a wasser, to remove a head, you need to remove the exhaust
and drain coolant anyway....so the worse is done, only an extra hour or 2 is
needed to check/re-seal the other one.

Now....this was the fun talk...any airplane mechanic will tell you the
following (George gave you a good answer on that), symmetrical/opposed/flat
engine are more sensitive about weight distribution / calibration...we all
know about that Wasser shaking problem... uneven compression can create
stress on rods and crankshaft and everything around + some shaky engine
reaction.

So before, you should take a few things in consideration; heads/engine age
and compression, oil consumption.

Fact: Lately, i did a lot of engine re-seal... most, not to say all engine
gain a 20 to 40% increase in compression after am done with them...just by
re-torquing the head, you mess with everything on this side.. imagine a new
head gasket (compression one), cleaned and cut valve seat, cleaned rings
groove.... so, if you had even compression before (or close), you may have a
very uneven compression situation now, one side is working harder than the
other one.

Another fact: as an example, one of the engine i did lately had:
130, 115, 95, 110 psi, not to bad.... the 130 cylinder had a lot of
carbonisation... after the cleaning: 125, 120, 125, 125.....
Conclusion, heads are everything, most pistons and sleeve i see have about
the same wear, not the head

Another thing, i have seen a lot of top broken ring (compression ring), just
a fact....

Anyway.... there is no real answer or solution...

Regards, Ben
http://www.benplace.com/head3/100_0025.jpg