If a meet is tied (whether dual or invitational/championship), the general rule is that the 6th runner breaks the tie - although, I can remember seeing some screwed up meet (Jr. Olympics?) where the 5th runner was used to break a tie. 7th runner can only displace other runners.
Also, if its dual meet and you are running more the 7 runners, only the first 7 can count (first 5 scoring and the 6/7th displacing). For example if you run against some horrible team and you place 1st thru 10th, then the score will be 15-50 (you win with 1-2-3-4-5 vs. the other team getting 8-9-10-11-12 no matter how far back they actually place).