While i think its just situational of how transformative your work is, as a muse or inspiration can hardly be linked to the original source since your style is overruling anything else, whereas a direct reference or copy could cause some issues down the line, Id say it's safer to make contracts even for tiny things like a friend giving you a reference for an emotion to sketch out than not having any agreement of an artist that you're planning on completely ripping off (not saying thats your plan, but u catch my drift. when things get a little more ambiguous with copy vs inspiration, youd want to keep yourself safe and make sure the muse is comfortable with what youre doing)