MongoDB Manual References vs DBREF
Short version: use a manual reference, not a DBRef.

Explanation:

There is no particular benefit to the use of a DBRef beyond giving you the collection and database that the referenced document resides in.  If you know the reference is only between categories and products, then that is not particularly useful.  A DBRef simply contains the _id of the referenced document, the collection name and optionally the database name.  You will usually have all that information in your application (i.e. you have the _id and you know that db/collection it resides in already).  

If that were not the case, let's say you had references to documents in a slew of databases and collections and it was difficult or cumbersome application side to keep track of which one each document resided in, then it may be a benefit to use DBRefs.  After all, the easiest way to do a manual reference in that case would be to store the database and collection that you are referencing in the document too, and that is what DBRefs do for you in any case.  

That (referencing multiple collections from a single source collection) is not a terribly common pattern though and not what you have outlined above, and so unless you have such a compelling reason to use them, manual references are recommended instead.

Some of the drivers have helper methods which may save a few lines of code versus a manual reference, but not all drivers do, and it may make the code harder to understand (without some nice comments/documentation to explain it).  As well as that, the DBREF approach is more fixed than a manual implementation - what if you want to reference a field other than _id for example.

Finally, in terms of product catalog design, I would highly recommend reading these two excellent and detailed blog posts from Antoine Girbal:


Part 1 - Schema Design
Part 2 - Product Search


There were also three presentations given at MongoDB World 2014 on this very topic, so I would recommend reviewing those also as you progress:


Presentation 1: Flexible, Searchable, Low Latency Product
Catalog
Presentation 2: Real-time, Geo-Distributed Inventory
Presentation 3: Scalable Insight Component (User History,
Recommendations, Personalization)