Until we get SERVER-863 keeping the field names as short as possible is advisable especially where you have a lot of records.
Depending on your use case, field names can have a huge impact on storage. Cant understand why this is not a higher priority for MongoDb, as this will have a positive impact on all users. If nothing else, we can start being more descriptive with our field names, without thinking twice about bandwidth & storage costs.
Please do vote.