As a tank, I can appreciate it when the THM/BLM take the time to sleep if people are taking unnecessary amount of damage.

I try to look out for it but if they sleep right as soon as I pull the mobs will wake up due to me using Overpower. So, I would say give a bit of a heads up or make a macro that alerts the party that you're casting sleep.

Likewise, when I go as my THM or even WHM I will frequently sleep adds if I notice the tank is taking a lot of damage. Another scenario where sleep is really useful is if people start to DC. Slept mobs can mean avoiding a potential wipe while the people in question return.

My best advice in regards to sleep, use your best judgment and do your best to adapt to the situation in each party. ^^