You would should not need stonith because HAProxy doesn't need a shared storage container. If your clustered pair go into a split brain situation why would it matter? Which ever one your router chooses to hit will work fine.
You will probably want the following in sysctl.conf:
#Allow haproxy to start without the interface present - Required for stick table replication:
net.ipv4.ip_nonlocal_bind = 1
Also make sure that your HA solution fails gracefully i.e. in a split brain scenario make sure BOTH nodes try and go live (At Loadbalancer.org we've used several patches to HA-Linux heartbeat to ensure that happens) However after saying all that a hardware implementation of STONITH would also work.