routing should be 'sticky' to an ip address; connections to any given ip address should use the same outbound route for the duration of that connection.

from http://wiki.openwrt.org/doc/howto/mwan3

Linux outgoing network traffic load-balancing is performed on a per-IP connection basis – it is not channel-bonding, where a single connection (e.g. a single download) will use multiple WAN connections simultaneously