Pre-2006 systems, maybe. I have a PIII based laptop that I'm reasonably sure doesn't contain any malicious microcode or BIOS shenanigans. It certainly doesn't have IME or equivalent. However, that was the CPU that started Intel's serial number controversy, and while I do have a BIOS setting to turn it off, is it really off?

It's a pretty deep rabbit hole if you really want to go down it. You can make a case for not trusting any CPU that you didn't design and fab yourself, and even then you have to watch out for your own mistakes and bugs that can be used against you.