If you have tolerance to opiates,then the diazepam should be safe.
I dont want to discredit that benzo or anything,but it is considered to have the most subtle and for that matter,non recreational effects among most benzos.
Its great to relax and chill out watching tv or something,but most ppl usually wont experience that fast,intense onset other benzos like Xanax or Flunitrazepam provide.
Id take 10mgs of diazepam for my first time but 5mgs more cant really hurt imo..