clippings from friends that sit on my desk under a 150W incandescent light
bulb.

With all those people on your desk, how do you get any work done?

occasionally a leaf will yellow and I'm wondering if this is due to over
watering or something more sinister like a fungus or insect.

No leaf lasts forever. If only the oldest leaves turn yellow, it is a normal
process.

I don't have them on any particular watering schedule and instead rely on
the wilting of the leaves to tell me that it's time that they want a drink

Do not wait for the leaves to wilt. English ivy, Hedera helix, likes to be
slightly moist. Water when the soil starts to dry out.
You can't post pictures to a newsgroup.
For further instructions on growing, go to rec.gardens.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)