Ive never found a directive or instructions on how they should be tied/attached. Having tried both I would care to guess the knot tie would probably be most likely. A few times we just bow tied it because at a given reenactment event there was need to change flags at a given point of the day due to change of portrayal on the field. Was difficult to get it snug enough not to loosen up or slide on the staff. Not to mention getting through thick brush and tree limbs that tend to snag anything and everything it gets a chance too, including bow ties that came loose at least one time. Using a knot it wasnt going anywhere soon.

As Ken mentioned Ive also seen staffs with a recessed groove at the top, a nail, screws, eyelet of wire etc. Assumed to better keep it from sliding down the staff. We also read accounts whereas the color bearer tore the flag off the staff in an attempt to prevent capture. If it had been bow tied once would expect that three short yanks and it would be loose... if knot tied wouldnt quite be as easy forcing it to be ripped off in haste.
Just an opinion and observation.