Visible sparks are not ok. I'm not familiar with your particular plug wires, but that normally means the insulation or wire connector is damaged.
If you see a spark, at a minimum it means that you're not getting a full spark on the actual plug because you're losing energy. It can also lead to premature failure of your coils because they are now trying to drive a spark through something that doesn't have the correct resistance.
Pull off the offending plug wire and, if possible, look for a tear or damaged connector. If you don't see anything, you might try to reseat the wire, and make sure it is positively connected to the plug and try again. It could just have a poor connection (not snapped all the way on).
You're probably looking at replacing the plug wires, and they normally only come in a complete set.