I think what you're really doing is a kind of "risk assessment". Risk assessment has two factors: one is the probability of a bad outcome, the other is the severity of the outcome if it occurs. The "product" of these two (subjectively determined) factors is your actual risk.
The probability of a bad outcome is sort of unknown here, but if you don't own the computer network at work, they are pretty much free to record whatever they want (if they really, really want to and if it is actually "worth it" to them).
If the worst thing that could possibly happen is that someone would listen in on your conversation with your wife about what time you're coming home from work-- then, I'd say your risk assessment is very low, regardless of what IM software you use.
If the worst thing that could happen is that your employer discovers that you're going to poach clients from them-- then yeah, that's a risk.