I'd be a lot more concerned about the sending and receiving chips and the quality of the cable or fibre when it comes to jitter, than any EMI or RF being introduced to the signal through the sound card.
But to tell the truth, I haven't had to worry about jitter or even think about it in over 14 years. This is an interesting exercise in remembering. (hmmm... what was my name again....???) Does anyone know if it is really a factor in any modern sound card setup?
The reason why it was such a problem in soundcards was because of the proximity of the AD/DA to the CPU. It wasn't about cable connections, because if you were using an outboard AD, you weren't suffering those kinds of problems. Yes, jitter was a much-discussed problem in long digital cable runs, but that was a completely different animal than the EMI emittence problems we experienced with having our ADC's a few inches from a super-high-frequency device with no magnetic shielding.
And, if we accept AR's anecdotal evidence, it isn't a problem in modern soundcard devices.