Definitely do as good of a sleeping bag as you can afford. It just helps ya sleep better, by not having to worry about the weather. Worse than a good frost, is an unexpected snowfall, in the mountains. That's when you'll be DARN glad you packed a tent and good sleeping bag. And you're more likely to survive, lol. 😁
Far better to have something, & not need it, than need it & not have it. 🙂