The ATA/100 cables are harder to round because each individual conductor is finer than those on ATA/33 cables. I round my own and can do the ATA/33's all day long with no problems. When it comes to the ATA/100's though I buy them already rounded. When I run into flat ones I just fold them over 4 times and wrap them in bright colored electrical tape.
I've never experienced any data corruption because of rounded cables. Frankly, I don't thing they have a huge impact on air flow or system temps. However, rounded cables do give your case interior a "cleaner" look. So, on the basis of form rather than function, I use rounded cables.