If you want to guarantee you get a seat that's not on level 4 (and level 4 is pretty terrible because the front half are all pre-sold) I generally recommend getting there around 4am. If you want particularly good seats or you're with a bit of a group get there earlier, if you're by yourself you can push it back. You can probably even get there just before the gates are opening at 8 and get a seat on level 1 if you're by yourself and willing to be a bit of a prick to people trying to hold seats. If you don't mind sitting way up on level 4 generally they're not all given away until 10 or 11. Things could be a little busier this year because of the Richmond factor but at the same time they're playing an interstate side with minimal MCC representation so I think it will roughly balance out.
Getting in at all should be no worries, there hasn't been a lockout in the grand final since 2000.