It is always better to be honest. Any level of nicotine user is considered as a smoker when filling the form for insurance policies. The premium charged from smokers will be slightly high. If you hide it and take the policy, alter when their is a claim, the agency can always deny it by saying that you have not disclosed the complete information while taking the policy. So in order to avoid issues in the future, it is always better to disclose all materials facts at the tie of taking the policy. Whenever a policy holder dies, there will be an investigation into the circumstances, and if the presence of nicotine is found, the insurance company has all the rights to deny the claim amount.
It is not safe to start smoking even after taking a insurance policy. Most of the insurance companies have contestability period, during which if something happens to the policy holder, and if it is found that the policy holder was a smoker, the company can deny the claim or else they will adjust the amount as you where paying the premium as a smoker and will only pay the rest to your nominee.
"Do not give up, things might not favour you always"