It's easy to fly in-balance in straight flight.
As indicated previously, as long as the wings are maintained level then use whatever rudder is necessary to prevent the heading from changing ie prevent yaw.
Apart from asymmetric considerations eg a power/speed combination that it has not been trimmed for, or banana shaped aircraft then the a/c must be in balance. If continuous rudder pressure is needed then the rudder trim needs adjusting.
Best way I've found is to use a speck on the windscreen directly in front of you, and align it with some object on the horizon (further away is better than closer). Use rudder to prevent your chosen speck from yawing to the left or right of the aim point. Remember this is while keeping the wings level.
If you need to turn the a/c to align the speck then a coordinated turn should be done.
Even a fairly well coordinated turn can be done just by looking outside the a/c.
Again, choose a speck, but ensure it is one that appears to be postioned on the horizon.
If you're using the correct amount of rudder then the a/c will appear to roll around the speck against the Hz before any heading change happens.
If you're using to much rudder during the roll to the desired angle of bank, the speck will appear to move below the Hz.
If too little rudder the speck will appear to move above the horizon & in the 'opposite' direction of the roll/turn eg roll right with insufficient rudder the speck will move to the left. This is due adverse yaw - the reason rudder was required in the first place. Because the roll is causing an angle of bank, the yawing motion won't be parallel to the Hz, thus giving the speck the appearance of moving above the Hz.
P*ss easy to demonstrate in a Cessna but not so easy in a Warrior or anything that has an aileron/rudder interconnect.
Hope this helps. I suspect your instructor is being a bit slack with his/her teaching.