Are you concerned about viruses/trojans on your Mac?
Do you use any sort of antivirus/trojan/malware detector? Or are you pretty confident that your Mac is secure.
So, yeah ... I use a detector, not only for the small(er) chance I get a Mac Trojan/Malware ... But also to protect anybody I do business with ... Well more to protect anybody in general.
Extra benefit, it keeps my system clean(ish) of tracking cookies and other privacy invading nasties.
The nice thing about OS X is the simple way it handles installing applications and if they ask for elevated permissions. I know that there are vulnerabilities with Macs, but I really didn't have an issue on Windows either because I paid attention to what I was running and where I was surfing online.