I use premium only in my 3.7l. Say what you want about wasting money on the extra octane, but working in the OEM auto industry for quite some time and seeing some of the R&D on engines, I will definitely continue to use 91.
Ignoring compression ratios, my general rule is 4cyl = regular, 6 cyl = mid or prem, 8+cyl = prem ... and than higher compression = higher octane