If you're asking about the effect of smoke breaks on productivity, I'd say it's detrimental.
Any time spent away from the counter on things other than the job (with the exception of bathroom breaks and lunch -- if you can break away long enough for one!) means productivity suffers.
From a financial perspective, if your store offers health benefits for employees, you're likely paying higher rates because smokers are frequently more ill than non-smokers.
In addition, some states have enacted laws that require smokers to stay well away from the entrances of businesses. That could pose a problem if a customer complains about inhaling smoke upon entering the store.
Some employers have instituted smoking fees on atop of the usual insurance premiums to discourage smoking.