Bear in mind that if you store nectar in a rack in a basement for a while it can become quite valuable.
Personally I think if you want to make money out of anything your Sim makes (painting, sculpting, nectar, whatever) then the Perfectionist trait is worth getting. Perfectionists don't take that much longer to do things. Although, like you said we'd need a confirmed ratio of extra time to extra money to learn whether it's really worth it, and I'd have no idea how to go about that. Also, I'm willing to be that this ratio would be different for each skill that it's relevant to!