I use Lookout. Not necessarily for its antivirus features, but because it makes locating/locking your phone easy if lost/stolen. I do have it set up to automatically scan every app I download and do a scan once a week Wednesday at 2am (when it'll almost definitely be plugged up and I'll be asleep).
But really, the chances of getting a virus are remote. If you're downloading pirated apps, maybe. If you're downloading shady apps, maybe. If you're the kind of person that never checks the reviews on a product and never checks to make sure that the app isn't taking more permissions than it ought to need, then maybe. But it's very, very unlikely... and unlike what most people say, Google does filter out malicious apps and will remotely remove them from your phone for you if something slips by.