Hello and welcome!
I've moved your question over to the Service Desk & Asset Manager (Powered by LANDESK) area because more Service Desk customers are likely to see it. I hope that someone will be able to reply who has a similar environment to you. In the meantime I found this past discussion which may also be of interest to you: Ticket Categories Help
Whenever I work with customers in this area one of the main drivers relates to the Management Information you wish to produce from your tickets. Will you be only using categories to categorise or will other attributes such as the affected service and/or configuration item be used?
Also, try not to think of categories in just the incident context. Consider introducing a category model that can be used in Request, Problem and Change etc. This way you will be able to standardise your reporting.
Also think about categorisation at Resolution (or similar) will you be capturing a category for example? Will this be the same list or will you have specific categories for reosltuon/fulfilment?