If I play Grand Strategem, does the +5 affect units played after that spell?

No, Grand Strategem does not affect units played after the spell resolves.

According to the ruling in FAQ #8163 and FAQ #8376, Grand Strategem only applies its +5 Might bonus to units that are in play at the moment the spell resolves. Units that enter the battlefield after the spell has resolved do not receive the bonus.

This is because the card uses the word "give," which indicates an active effect that applies a snapshot of the game state at the time of resolution, rather than a continuous aura effect.

Sequence of events:

1. Grand Strategem is played and enters the chain.
2. The spell resolves.
3. The +5 Might bonus is applied to all friendly units currently on the board.
4. The spell is placed in the owner's trash (as per Rule 153).
5. Any units that enter play after this point are not affected because the spell's effect has already been executed and concluded.

As noted in Rule 153, a spell creates a game effect according to its instructions and is then placed in the trash of the player who owns it. Because the effect is not a continuous or passive ability, it does not persist to affect units that enter play later in the turn.