Where the new units go
Every level adds a slot, and the new unit changes the board around it. Give it the hex its job asks for, then check your carry is still covered.
Checked for patch 18.4. The numbers come from the live Set 18 data and follow every patch.
The idea
Every level adds a slot: 8 units at level 8, 9 at level 9. The new unit is more than its stats. It takes a hex, and that hex decides whom it protects and whom it leaves open.
Top four boards at level 8 usually play 5 frontline units and 3 in the back line. On average they go from 4.6 frontline units at level 8 to 5.2 at level 9, and from 3.2 back line units to 3.4: more of the new slot goes in front than behind. Which line your next unit joins depends on what it is, so decide its job before its hex.
A new tank goes where the front line is thin: the weaker side, or in front of your carry if nothing stands there yet. A new damage dealer goes in the back, usually apart from your carry, so one area hit or one leap cannot reach both. Then look at what the new unit changed: a wider front line may leave no weak side to cover, and the board across from you may call for a different side than it did a level ago.
See it live
A current comp on the 3D board at the level most of its games end on. Step up a level to see the unit it adds and where its boards put it, with the front and back line counted against top four boards.
Rapidfire Kayle Xayah carries with Kayle, a 2-cost. It is the most played comp whose boards add a frontline unit and a back line unit as it levels. Each level's new unit stands where its boards put it.
Level 7: 5 front line, 2 back line. Top four boards at this level usually play 4 front line and 2 back line.
The rule of thumb
Give every new slot a job before a hex: tanks to the thin side of the front line, damage dealers to the back, apart from your carry.
Check yourself
At level 8, Rapidfire Kayle Xayah adds LeBlanc. Where do its boards put LeBlanc?