I usually shift when I hit the limiter

7200 rpm ftw. Sometimes I shift up at between 6-7,-000 rpm. You basically need to find out when your engine runs out of puff and then shift up just after that.
Zetecs usually run out of puff about 5500 rpm (i think) and thats when the most power is there too. So your shifting quite early really.
The higher rpm you go to, the more engine speed you carry into the next gear, so you will effectively get more speed more quickly in the next gear. But with the 1.6, after about 5500rpm. But you dont want to be sitting in the same gear thinking 'i need to hit 7000rpm' if its going to take an age, because thats time and speed lost in the next gear.
You wont do any damage to the engine reving it high, engines are designed to do that, and the limiter is there to stop it over-revving and things going wrong.
Also, if you have a little search of the forum, this has been asked about 48,972 times lol