Yeah, it's probably best to crash it into a huge atmosphere than letting it hit ANY of the moons. It would spread not just microbes but radioactive debris. Cassini is not something I'd like to see wander the solar system as space junk.
I'm not sure if you're aware of this, sieur, but space is one big seething ocean of hard radiation. In fact the #1 danger to any astronaut crossing space to any object even in the inner planets is hard radiation.
See, the only thing protecting Terra from becoming a barren lifeless frozen rock like Luna or Mars is the Earths electromagnetic field (ionosphere), which channels most of the harmful particles and high energy EM light that rains down on the planet, into the electromagnetic lines of force instead allowing them to hit the surface.
Radiation from Cassini would be comparatively minuscule to that surging around the solar system from the solar wind (mostly protons) and cosmic rays.
There is some concern however about intermediate zones of the larger outer planets where theoretically conditions might be someone milder than the upper atmosphere or the lower regions. In the clouds of Saturn/Jupiter, it is conceivable that some sort of life might exist. Highly unlikely but not impossible. So caution might be warranted with biological contamination.
Radiation from plutonium isotopes in Cassini are not a real concern because the volume of those planet's atmosphere are inconceivably massive.
