It's up to you, really. Just choose the convention which matches your needs.
In our team we usually use "CR-<JIRA project key>" naming scheme. Thanks to this it's easy to map a Crucible project to a JIRA project. And at the same times we don't mislead JIRA issue keys (e.g. JIRA-123) with code review IDs (e.g. CR-JIRA-123).
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