Do you collect feedback on your interview process from software engineer and developer candidates? Do you focus on the people you hired, or do you give everyone a chance to weigh in?

We do not have a process of collecting interview feedback. But I see a merit in this as it would help make improvement to the process to get better shortlisting and selection. 
Once part of the team, all members are equal and hence to treated at par.

Historically we have not collected feedback. 

Interviewing is a skill and it’s not an easy one to master. We do have processes in place to solicit feedback from interviewees. This allows the candidate the freedom to provide specific feedback on a specific interview or provide overall impressions from all the interviews attended in that session. Typically, this is done as a follow up task post the interview by the person who scheduled the interview. Every candidate , regardless of the interview outcome, has the opportunity to provide feedback. Constructive feedback in used to make adjustments to our interview process.

We do collect feedback from both hires and non-hires.  I don't think the feedback is candid enough.  I personally watch LinkedIn, Glassdoor, and LAYOFF.COM to see what the blunt, unfiltered and perhaps FALSE, comments are.  All feedback is valuable, but you need to consider the source, and determine if you will use the feedback in a helpful manner.