I may be in the minority but I get in early and sit in my designated seat.
One of the reasons is that I have my elderly parents with me so want to make sure they are settled.
The other reason is to avoid the mad rush of pissed up twats 5 minutes before kick off trying to find seats all together.
So, if you want your guaranteed seat get to it about half an hour before the kick off. If you are happy to take a risk, anytime.