I think of rumors as a way to get the players info and something they might be interested in checking out. I agree that if most of the rumors are lies then the players might tend to ignore rumors so I would lean towards most rumors being partially true, some being completely true, and leave out lies. I don’t think there’s any benefit to having lies in your rumor table. One could argue “realism”, but I don’t think that’s a good reason.
For me, the half truths are more interesting so maybe 80%/20%.
I think lies always fall flat in play. They’re a waste of time. As a GM, I try to only pass on rumors that contain at least a nucleus of truth, some kind of hint that may become useful later.
I don’t tend to include outright falsehoods among my Rumors, there’s usually a grain of truth to each of them, but it might be a bit distorted through re-telling or exaggerated sometimes.
I find the best ones intrigue the Players by sometimes contradicting their expectations or contain sometimes tempting inconsistencies. “There’s a Wizard in the Scabrous Swamps that can cure any known Disease” is like this (in a Setting where Wizards don’t have Healing Magic) because it plays with some of that knowledge in addition to being something that might be “useful” later.