It is common practice for the free bet to be given after the bettor places their initial wager, and the value given determines on the type of free bet rewarded. At that point it is up to the client as to how they will use the free bet. Some may choose to bet cautiously in hopes of adding to their existing bankroll while others might feel a bit more daring. Given that it is “free”, some are simply trying to reach their rollover limit to cashout quickly.
Either way you choose to play is up to you, but do note that it is also common for online sportsbooks to require you to risk all of your free bet value at once, rather than spreading it out over many individual bets, though this is not the case with traditional sportsbook cash/keep stake bonuses.