Depends on your processes to be honest.
If you've got development teams that run themselves, you probably want to let them generate their own customised workflows (suggestion - look into Simplified Workflows that the Jira Agile add on provides). If you need your teams to follow common business procedures though, you'll want them to work in similar ways which means centralised committee (although if you're going to do this by committee, keep the committe small and close to the users who will end up with the workflows)
It's even fine to mix the approaches - a place I worked had one group that was doing Incident management, meta-project work, support, etc - they needed fixed workflows that enhanced their reporting and matched the real-world process. They then had several groups of developers who were delighted when we got them simplified workflows and told them to do it themselves.
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