Been hit a couple of times recently, so I'd say they are definitely checking everything.
Had a time about 6-8 months ago where 9 out of 13 items got hit (everything kinda bunched up with delayed items being released and other on time) but before Christmas things sailed through untouched.
Definitely seems like they have busy and quiet periods. If they don't have a big work load I suspect they have the time to properly check everything while if they are busy they probably let a few things slide to keep up with the work load, or they just glance at the declaration and don't spot that it should be hit with customs because they are trying to clear things.