Level up to 4 early if you want to start a win streak. Level up to 5 to try and continue the streak.
If you're weak in the early game, you can opt to save gold and lose streak instead.
At Round 3-2, level up to 6 regardless of your start. Put together the strongest team you can.
In the 1st round, go up to level 7. If you are weak, lose a few rounds to stabilize your build. After that, go up to level 8 and complete the build.