A lot will depend on teams. If you have backrowers who can play centre than you could probably carry another prop, or if you have a 60min hooker you will definitely need to have a utility. Each team is different, but i would most likely think the ideal bench should be something like...
A utility player, most likely a back that can slot into dummy half. - Lance Hohaia
A prop forward, an impact player more so than a work horse. - Fuifui Moimoi
A prop that can slot into backrow. - Tom Learoyd-Lahrs
A strong backrower. - Neville Costigan