If the reason your not offering someone a job is because they messed up technical questions, I think its a good idea to give them candid feedback on the spot. Its objective. If you are having to choose among several qualified candidates, then I think this is asking for trouble, since there isnt an objective clear reason for the person not getting the job.
The places im interviewing are are the opposite of startups. One was a community college, very rude .. when I asked why I wasnt notified that the position was filled she said "im planning to send out an email". No email to date and this was a 3 weeks ago.