Using multiple virus scanners - overkill?

[otctrader]

My default virus scanner for years has been Norton, but with my subscription expiring I began looking at the competition - it seems as if the in vogue scanners these days are NOD32 and Kaspersky.

Anyhow, to the point, I ran Kaspersky's free online scan and found a few items (mostly "seedy" downloads of mine) NAV never detected, and also found a few things detected by NAV that Kasp didn't find (usually NAV's generic Bloodhound detection).

Beyond having one backbone virus scanner running in the background, is it overkill to run several scanners on suspect downloads, and will having three scanners on your machine (although only one in "live" mode) cause any conflicts?

Also is there a good guideline for detecting false positives, i.e. Kasp and NOD32 give clean bill of health yet Norton flags the file as a Trojan? Who do you trust?