On my trainer and low-wing trainers I had them balanced to a slight nose down attitude when checking the CG. There's the old saying "A nose heavy plane flies bad; a tail heavy plane flies once."

Having a slight nose-down attitude means if you lose power, the plain will want to gently dip forward. This will allow more ability to keep airspeed and air flowing over the wings, creating lift.

Either way, like they said above it's not THAT critical. Get it as close as you can, and make adjustments based on how the plane flies. The manual is just a safe starting point.