Definitely do snow tires. Chains are cheap, so why not carry them? It's pretty rare, but we do get R4 and even R5 conditions every few years. Though if they say R5, I'd just find a hotel or place to hang out, because the road is going to close. Those are awful conditions to drive in.
When you hit those conditions, chains are required for R5 and for a FWD vehicle I think they are required for R4. The only other reason is if you get off a plowed road, you might get stuck and need chains to give you the extra traction to get out. Also, throw a snow shovel in your trunk too. You never know when you might need to dig to get out of a place you parked, or slid into...