From this NASA link [1] it looks like when Cassini crosses through the plane of the rings, it does that almost at a right angle. The width of the rings is at most 1km, and at a speed of about 20km/s, each crossing will take at most 0.05s. The average distance between rocks in the rings is a few meters (and Cassini is about 4m by 6m), so even if Cassini crossed the rings themselves the probability of a crash would be about 50%. The average distance between space debris in the space between rings and between rings and the planet is probably hundreds of times lower, so I would think that the probability of a crash is very small, easily less than 1% per crossing. For Voyager, a crash probability in the neighborhood of 1% was most likely unacceptable, but for a spacecraft scheduled to be destroyed on 15-Sept-2017, this is probably ok.

[1] https://saturn.jpl.nasa.gov/mission/spacecraft/navigation/