At peak weight everything is cinched down tight and the bow should performmost efficiently, quietlyand transfer energy best. With that said: I don't think you lose all that much operating efficiency or that much in the energy transfer (depending on the arrow weight) having a 70 backed down to 65. I like the versatility of having a 10+ pound range to adjust in for using different arrow setups but that is not necessarily for everyone.

EDIT: If you use the same arrow setup most, or all, of the time then the 65# bow would probably be the way to go.