This is just my opinion others may have a better strategy.
In the beginning I always reseted as soon as possible, use the Mojo to buy Charms in Crafts, this will help you reach higher levels faster.
But Look out for the Affection you can only earn 200 every 24hours so if it's full maybe wait with the rest to not loose any.
Later you have to reach a minimum of levels to be able to reset but if you are close to unlock a new girl keep playing and reset after you have unlocked her, the new Girl will make it easier to reach that level faster.