I would be interested to hear how other users are using tags and fields in their TiddlyWiki's?
In my mind they can both achieve the same thing though I'm thinking tags are more useful for grouping based on the content of a tiddler, whereas fields are more properties of a tiddler regardless of its content such as status.
For example I am currently using TiddlyWiki to document an SQL Server database. I am using tags to group the tiddlers into categories such as Tables, Views, Stored Procedures etc.
I had been using a "Complete" tag to mark the tiddler as being documented but I feel the field "status" is more appropriate for this.
Equally I could use a field for the categories though I feel that having this information hidden is not appropriate.
I guess there is no real "right" way to use the combination of both.