Nah, that won't pull the nose off heading unless you continue to pull for too long after you start the roll. You can cheat the nose up just a little at the start of the roll to help maintain altitude during the roll without it being too noticeable - assuming you're flying in front of judges.
How you trim for acro is a matter of pilot preference. If you're flying something like the Great Lakes, and are trimmed for zero G, you'll find that it takes a little less effort pushing through the inverted portion of a roll, but you're also going to need to be holding significant aft stick pressure when flying upright. For Great Lakes style acro, you'll spend a lot more time flying positive G than the momentary negative, so you must figure out what works best for you. A common trimming technique for "Sportsman" level acro is to trim for the speed you'll be flying upright as you exit a maneuver. This prevents you from having to fight stick pressures between figures. This is actually how I still trim the Pitts for acro, even though I'm flying Advanced sequences with more pushing. But the Pitts is not nearly as heavy on the controls as the GL. Other pilots flying Advanced and Unlimited sequences trim for zero G. Personal preference. I once flew a Great Lakes and it does require a fair amount of force to push negative G during a roll if you're trimmed for positive G. I assume your instructor was attempting to alleviate some of that pressure by trimming for zero G. If I was flying the Great Lakes, I'd trim for my upright exit speed, and just use two hands to push during a roll.
