The goal of an Assassin strategy is to get either three or six Assassins as your main carries. They are high-priority for upgrading to three-stars and receiving high-tier offensive items. The rest of your strategy usually needs to provide utility for your Assassins, usually in the form of crowd control or possibly healing.

The fact that you want multiple units on a three-star level means that Assassin strategies rarely want to go to lvl10 – you need gold for rerolls, and your highest priority Assassins cost 3 gold, which means it’s most likely to find them when you are lvl7 or lvl8. I.e. Assassins are an upgrade value strat.

That said, since the introduction of Void, it's a great idea to try to reach at least lvl9 after finding one or two key upgrades on lvl7 and 8. This will increase your chance of finding the Assassin Ace, which makes a big difference in the late game.