Heuristic: If we could change it to 'zebra' on a whim, then use a string; otherwise use a symbol.
E.g. Use strings as keys to the params hash, since changing flexible route segments, query string parameters, or name="" values is easy. Use symbols when referring to actual column names.
Use Hash#fetch instead of Hash#[] (for better error messages when wrong key is supplied).
To be consistent with the above, use Array#at instead of Array#[] (even though they are identical).
