When I started learning how to configure email, SPF existed but there were doubts about whether it was a good thing, and the value of offering SPF records in DNS. Now it seems that it is widely accepted that some form of well-known sender validation is good practice.
Is this really true? Am I being a bad postmaster by not supporting SPF/DKIM/whatever?
You are a bad postmaster if you have never evaluated the costs/benefits of adding SPF and DKIM. If you have looked at them and decided they're of insignificant benefit, that's your decision, and I highly respect that you made an informed decision.
Mailservers I configure are set to strongly enforce the policies you set via SPF & DKIM. Server software that supports validation is usually higly configurable; you can enforce the sender's policy as-is, add to it, or use it as part of a multilayer approach (such as SpamAssassin).
I always publish SPF records, as it's very quick and easy; DKIM take just a little bit more, but does require software that supports it.
As far as sending mail: sender validation is only important if the receiving mail server is going to check for it. A few years ago this was scarce, but nowadays I see many more mail servers actually validating senders, with a lot of them using SPF.
While I do have SPF setup on my mail server/domain, I don't validate senders when receiving mail because I work with many smaller corporations, who do not do any sender validation, yet I need to receive their email.
My $0.02: I'd eventually do sender validation because I think it's a good way to go, but there's no rush.