It's all dependent on how much risk you're willing to take. For me, I consider the following before deciding whether to reserve or not:
1. Distance driven to get to that destination. The farther I have to go to get there, the less likely I'm willing to take a chance on not getting a spot.
2. Number of campgrounds in the general area? The more campgrounds in the area, the more I'm willing to take a risk.
3. Importance of the campsite to having a good time? If the plan is to spend time at the campsite, then I am less willing to take the risk. If it's just a place to sleep at night, then I'm more willing to take the risk.
4. Popularity of the general area? And, is it seasonal or year round? If it's extremely popular and there is no "off season", then I'm not going to take the risk. If it's open year round and I can go off season, then I'm more willing to take the risk.
For me personally, for Yellowstone, I'd get reservations. The expense is not a deal breaker.
