Ok, so this is something that I see a lot in pubs ? people just don?t know when to level their courier up and fall behind as they get swarmed by more informed opponents.
Each round you get 1XP, and you can buy 4 XP for $5. Which means that buying XP jumps you ahead FOUR turns in the game. So if you look at the point where you would ?natually? level up, you can predict back four turns and level up early by buying XP. Buying XP is great because it lets you make a stronger board, but MORE importantly, it unlocks the more expensive units faster. You?re never going to get a T6 Lone Druid or Kunkka without buying the XP to level your courier up.
Edit: (tip from tundranocaps) The above advice is to maximise buying XP so that you can put out an extra piece that turn. If you want to increase your odds of being offered expensive units the next time you enter the prepare phase, you should buy XP so that the natural round XP you get is the one that levels you up. This means?almost?levelling up on turns 4, 8, 12, 16, 20, 24 etc instead of 5, 9, 13, 17 etc.
At the start of the game, levels come easily. On turns 2, 3 and 5 you get an extra courier level without having to invest anything. But on Turn 5 (T5) you can invest $5 if you have it spare and jump straight to 5 units. I actually usually buy the 5th level on T6, since money is such a scarce resource in the early game (you are investing in pieces willy nilly trying to get ANYTHING to 2*).
Edit (from tundranocaps comment): a new strategy that?s been spreading is to not level up actively from 4 to 5, unless you?re really strong, as that increases the likelihood of you seeing more of the units you?ve seen in the first 5 rounds, and thus increasing the odds of 2* units. And then you level up as soon as you get one of those.
Level 5 rule: Level up by T6. Sometimes you can do it on T5 or even T4, or even deliberately hold off to fish for more $1 pieces.
The key thing about levelling up is that every XP you buy means that future XP purchases bring you closer to the next level. If you buy XP to level up to 5, then you only need to buy XP once more on T9 to hit 6th level.
Level 6 rule: Level up by T9 at the latest.
The jump from 6 to 7 pieces is a big one, and using our ?rule of 4? we can see that it is either going to happen on T13 where you pay 3 times, or T17 where you pay only twice. I generally would go for the aggressive early level if I am STRONG, riding a winning streak, and I have good units on my bench that I actively want to put onto the board (I might also reroll a little to preserve this winstreak, although I try not to). If I have just lost rounds 11 and 12, I usually use rounds 13-16 to build up a losing streak, and only level up on 17. That gives me the most economy for my position. Alternating wins and losses is the worst. You are far better of recognising when you are weak or strong and playing for streaks, and the teen rounds is where this is most important.
Level 7 rule: Level up 3 times on T13 if you are strong and want to win more. Level up twice on T17 if you are weak and farming a losing streak.
From there the next two points to level up are 21 and 25. You should go for level 8 on Round 21, after banking all of your gold on the creep round 20. You can also do some light rerolling from rounds 21-25 (interest permitting) since the $4 units you see are what decides your late game composition and you want to start getting them to 2*. And while it makes mathematical sense to get to level 9 on T25, because it?s a creep round I would just hold my gold for the interest and level up on T26.
Level 8 rule: Level up on T21, and do a little light rerolling to assemble 2* $4 units.
Level 9 rule: Level up on T26.
Once you are at 9 units, you have a decision to make as to whether you are going to start rerolling to make the best 9 unit board you can, or whether you are going to try and build your economy up to get enough interest to buy the 10th courier level / chess piece.
Ironically, your strategy when determining whether to reroll or not is the OPPOSITE of what motivated you in the teen rounds. There you were rerolling to make a strong board and continue your winstreak. Here if you are losing you are desperate. You can?t afford to ride a losing streak as you will promptly be eliminated from the game. So it?s the WEAK players who are rerolling on 9 units and have given up on getting the 10th piece. If you aren?t saving for the 10th piece then it?s ok to burn through all of your gold making your board as strong a 9 unit lineup as you can. Conversely, if you already have a strong lineup as you reach 9 units, now is where you get greedy and hold off, building back up to $50+ to get to the 10th level and your final form.