I advise against it. In my research group (social sciences), we have post-doc opportunities, but we have very targeted searches and only consider candidates when positions are open. I receive lots of unsolicited applications throughout the year. Many are not remotely close to our area of research, so it seems to be a SPAM-based approach. The only time I think it might make sense is if you brought funding with you, and the post-doc was relevant to the broader work being done within that research group. That might be attractive to a research group when a funding source is not immediately available to hire a post-doc.
If you are interested in a given University / research group, you may consider reaching out to see whether post doc opportunities are on the horizon. That will likely be a better use of your time.