As has been said, there is no right answer to this. It's all about managing risk, and deciding how much risk you want to live with.
The last time I got a virus was back in 1988 on a DOS machine. For a good fifteen years now I haven't used any security software at all apart from what comes with Windows. No problems at all. Until a few years ago I would go every six months or to the Kaspersky site for a scan, just to be sure, but it never, ever came up with a positive.
I think the risks are tiny, with my particular usage pattern. If you download lots of dodgy software, visit questionable sites, and like to open unsolicited email attachments, you might consider yourself to have a higher risk.
Beware of those who give a categorical "yes" answer (just look up the thread slightly!). Such categorical answers cannot be justified, and I'm here to prove it: my opinion is based on over 25 years of virus-free computing, with no additional protection for at least 15 of those. The only legitimate answer anyone can give you is "It will reduce the risks, but it's up to you to decide how low you want the risks to be." For me, they are already plenty low enough.