could not find any stratigytips on this. but general rule is talk, at least when talked to.

if you are planning to fight for second, tell it to the leader.
scenario : three players at A:12, B:8 and C:9 fullstacked terretories.
leader has a good lead, but aren't usually that comfortable.

player B says, I am going to flag, going for second.
player C is silent. (but planning on fighting for 2nd)

most cases leader will attack player C to preemptive strike him, instead of letting him fight for second.

PART 2:
whenever you are flagging, tellit in the chat:
PLAYER: flagging for 3rd.

do this for two reasons;
people are often more concerned with keeping an eye on the chat than the acctual place the flag comes up.

second reason;
there will often be a lagg in the system preventing the flag to go through, I once had to press the flagbutton 5 times before it wnt through it is easy to just flag and then assume it goes through.