I ran an MTA with a self-signed certificate for a couple of years, until real ones got cheap enough that I could no longer be bothered to do so, and I didn't have a single rejection because of the unsigned certificate in all that time. I never had a single complaint about a mail being marked as spam because of it, either; if anything, using TLS often seems to mark you out as a non-spamming professional.
In my opinion, it is definitely worth enabling SMTP TLS if you can, whether or not you pay for a third-party-signed certificate.
Edit in response to your comment: It's not that someone couldn't choose to restrict inbound mail on that basis; I've never come across it, is all. A third-party-signed certificate is still useful to prove there is no man-in-the-middle attack happening; but that doesn't seem to be a serious problem in the MTA world at this time. If that starts to change, we could well find that people start to insist on signed certificates.
Security exists to address threats, so if the threat model changes, the range of sane and proportionate security responses will change with it.