The Associated rear hubs aren't side specific, so there won't be any toe whatsoever. The Losi ones might have some toe built in, so there would be a left and right. Then it would just be a matter of switching them around.
If you want to do some serious research on rear-toe for oval racing, go to custom works website.
http://www.customworksrc.com/aa_support/support01.php
Many of those setups are asymetrical. For example, left rear toe will be in the 2-4 degree range and the right rear toe will be in the 0-2 degree range.
Not sure why you want zero, but it makes for some interesting reading...