A crown would signify rank of some sort. Be it a simple circlet or more impressive depending on your rank and standing. I would suspect William would have frowned heavily on anyone wearing a crown in his entourage more impressive than his own. I generally try to avoid them on rank and file, and if I were to leave one on a unit commander I would shave it down to a simple coronet/circlet.
Even at this early date we know the Holy Roman emperors struggled with the papacy over who could crown royalty, and legitimacy was not assured if you failed to do it correctly. William also received a new crown upon conquering England, and had to go through a new ceremony to achieve legitimacy. The Normans in Italy had troubles achieving legitimate titles and rights until Guiscard, implying an unwillingness to legitimize how they garnered their 'crowns'. They may have worn them, but titles were not fully conferred until Guiscard.
Still, they are your minis and if you like them, keep them. It is possible minor nobles wore something on certain occasions.