Hippie shouldn't be an offensive term, however in this era, it definitely could be construed as such. Most people that consider themselves hippies are happy and health conscious. They also care about the earth and being close to Mother Nature.
Unfortunately, certain people think of hippies as a person who stays drugged out, without a job, probably smells bad, and doesn't eat meat. This stereotype isn't true and is quite ignorant. So, no, don't take offense to the term hippie.
Personally I would not take offense if someone called me a hippie. I happen to associate with the culture of hippies however. Others might take offense to the term, especially if they do not have a favorable opinion of hippies. Some of the negative stereotypes about hippies is that they are dirty and smelly.
Some people also might think they are privileged because they live a lifestyle that allows them to not hold down a job or be free of a society that demands a more rigorous schedule. It is also a common stereotype that hippies smoke a lot of marijuana and take other drugs.