I tend to use 50% as a soft target, which seems like a good compromise. Sometimes it may go a little bit over that, but if it's occasional it shouldn't be an issue.

It's not good to go much over 50% on a server (assuming half the cpus are just hyperthreads), because you're essentially relying on your load being able to share the actual cpu cores. At some point, when the load increases too much, there may not be any headroom left for sharing those physical cpus. You then get to the point where adding a little bit more load to 80% suddenly results in 95% utilization.