Actually, most tanks worth their salt already have reached quite high levels regarding the warcry structure bonuses. They have already gained these bonuses which means you will receive them no matter what you do in their group...
By not using the skill in question, you are simply giving these creatures thousands of hit points worth of fight rounds. Whether that is a smart choice is entirely up to you guys but I certainly know what my opinion is
For newbie/lowbie tanks, that will be a tough question. Are players honestly going to pass up good exp by not using deathblow to help build a tanks warcry stats? I would be quite impressed if that happened because most players quit groups when the exp sucks...
Personally, I hope you guys choose to stop using it