I convert to string in code to compare and I ensure that anything that looks like an ObjectId is actually used as a ObjectId.
It is good to note that between the ObjectId (http://docs.mongodb.org/manual/reference/object-id/) and it’s hex representation there is in fact 12 bytes of difference, the ObjectId being 12 bytes and it’s hex representation being 24.
Not only is it about storage efficiency but also about indexes; not just because they are smaller but also since the ObjectId can be used in a special manner to ensure that only parts of the index are loaded; the parts that are used. This becomes most noticeable when inserting, where only the latest part of that index needs to be loaded in to ensure uniqueness. You cannot guarantee such behaviour with its hex representation.
I would strongly recommend you do not use the ObjectId‘s hex representation. If you want to “make your life easier” you would be better off creating a different _id which is smaller but somehow just as unique and index friendly.