Main source of Trojans will be pirate sites where you can download Free software, but can also be sneaked into other app markets. If you not in the habit of installing software from other sources and only use Google's Android Market then you dont need an Anti-virus.
Yes there has been some rouge apps found in Google's market but they removed it from both the Market and the users phones that installed the software. They have since then started to screen applications for known bad code.
Personally I install an Antivirus not because of it checking if apps I install are safe but mostly because they also give you options to remote back-up/locate and wipe your phones when stolen or lost
See for example findr service from
Avgfree Web site:
http://www.avg.com/us-en/antivirus-for-android
Or the Locate feature from
Lookout Also a great app is their
Plan B that you install after the phone is lost
Web Site:
https://www.mylookout.com/
Both of the above software lets you log into a web browser and remote manage your phone, personally I prefer AVG.