I help run a label(Raund Haus) and I can say that my favorite part is hearing demos and getting to listen to music before it is out. I imagine most people that run smaller labels have a similar opinion, as they wouldn't be doing it if they didn't love listening to music. I'd say don't be scared to send demos to labels that you think will fit in terms of style and quality. Don't send some generic email, definitely research the label enough to know that you'd be a good fit and explain that in the email. Much like writing a cover letter for jobs, if that shit is generic, your resume might never get clicked on; in this situation, the demo might not get listened to. Maybe that helps!