You might want to check your Time Machine backups by hand from time to time using the tmutil[1] tool.

My Time Machine backups in the past have been missing gigabytes of data, without me telling anything about it. And not just volatile data, like temp files or caches, but photos, music and documents.

[1]: http://osxdaily.com/2012/01/21/compare-time-machine-backups-...