absolutely. You are buying insurance....they are taking a chance that you won't die by doing something stupid AFTER you sign up. That's factored into the premium you pay.
The questionaire you sign is a snapshot in time. The question about smoking states 'have you smoked in the past 36 months?'. If you said no, and were being truthful, and then started smoking the day after your blood test, and died of a smoking related illness before you had to re-up your policy (new test), the claim will be paid, no questions asked. There may be some litigation, but the contract you sign does not state that you must keep your agent updated as to your hobbies, health, cholesterol levels, drug use, or anything else. Another interesting fact is that if you take out a policy, then kill yourself, it will be paid in full, but only after a certain amount of time from the date of the start of the policy, usually 2-3 years. Why 2-3 years? I'm sure it's based on statistics and equations that someone calculated.
Again, it's all factored into the premium. They play the law of averages; most people that don't smoke are not going to take it up, just like most people that do not do drugs are not all of a sudden going to start smoking crack, at least the ones that buy insurance policies for their families.
If you paid into a policy for 20 years at 2K/year, and then you were denied a claim because after you opened your policy you started smoking and died, well, that would be theft by the insurance company, and you could sue them, and you'd win. And I think that is illegal, at least in CA where I live.