I have been adjusting a few headsets recently and I have been using an incremental approach where I adjust the adjustable race and tighten the lock nut and then check for play etc. I find that sometimes when I try to loosen the lock nut to readjust the headset, the stem and handlebar get pushed out of alignment (usually because they get used for leverage or are obstructed so they cannot turn with the headset).

The end result is that while the wheel is facing one direction the handlebar and stem are facing another. At this point I loosen the stem, correct its alignment, and then continue adjusting the headset.

With regard to this issue I have a few questions:
1) Should by stem be tighter so that it does not get pushed out of alignment?

2) What are the most likely adverse results? Can you make a steerer tube unusable by grinding too much of its inside surface through this slippage?

3) What is the best way to prevent this from happening?

The funny thing is that as a kid when this happened (usually after a fall or failed stunt) we would just put the wheel between our knees and twist the handlebar till it was straight. Now that I'm older I sometimes wonder how those BMX bikes just kept on going.