With broadheads that use the arrow's insert as part of the head's blade retention system - like Thunderheads - you do need what Easton calls UBAR's (universal broadhead adaptor rings). It's nothing but a fancy name for a tapered washer that goes from ACC diameter out to the ferrule diameter of the head and gives the blades enough support to keep them in the broadhead.

Basically, if the blades fall out of the broadhead when it's not screwed onto the arrow, you need the UBAR adaptors. With heads that have their retention system independent of the arrow - like Muzzy, for instance, or solid cut on impact heads like Steelforce - no need for anything extra.