I did when I first got my phone (Lookout), but after researching the necessity of it or lack thereof, I uninstalled it about a year or so ago. I've never had a problem.
I read this from redmondpie.com
"The fact of the matter is, you, the average Android user, are safe from mobile malware. Malware on Android does not spread like it does on desktop computers because there really isn?t a simple method for them to spread. These viruses, trojans and rootkits that you hear of all the time only reach those users who do things like downloading APKs from untrusted sources and downloading bogus apps".