When to go on Ketogenic?

I am curious as to when some of you gurus would recommend someone starting a Ketogenic diet? 25-30lbs over? I believe in pretty much the guidelines the cutting sticky has detailed. Are keto diets typically for the obese? Or is it common for a lot of bodybuilders to jump on this from time to time? Would this be good to sort of "shock" your system every now and then (but I guess that would be more of a CKD). If one is going to be going full keto, would the cardio be "AS" important as just following a normal cutting diet? Anyones thoughts would be cool.

i thot atkins was the best thing when i was on it for 3 weeks at first i was feeling better etc... then i felt like s*** added carbs only good carbs tho and felt alot better since then i hate fake foods(choc cakes etc.....) and stick to natural foods wether cutting bulking maybe it did shock my system cuz i did like to eat crap now i hate em