Went last year.
Tbh, how you go it depends on you. I know people who practise every lunch and still screwed up on the day. Time is not an issue and there's more than enough chemicals to redo it. Also, make sure at least one person knows how to do the calculations (one of my friends totally forgot :L).
For my group, we wanted to back out last minute but our teacher guilt-tripped us and we went along. We just did what we normally do... we all had to go somewhere after so we kind of rushed it. We handed our results in and left a few mins earlier.
Came top 30 in the state LOL with like 4%, 15% and 24% error each. So as you can see, anything could happen on the day