That is exactly the meaning of normal practise: Most of them do, some don't. In even more organized airlines, it's the airline that puts rest times into the duty roster. Of course the captain is always allowed to deviate from this if he deems it necessary.
Let me assure that I do it the same way as you do it: I stay in the cockpit if it makes sense, after carefully considering the capabilities of the rest of my crew (i.e. my two FOs). When you fly e.g. from Australia to the Middle East, you have ITCZ almost on all your 14 hrs of flight. So you have to get your rest somewhere in tropical weather. It needs some kind of courage to trust in your crew, you have to sleep, otherwise you will be underperforming during the approach, but you have always your doubts, when you lay in your bunk and cannot sleep because of the Monsoon over the Indian Ocean - especially after we know about AF447.
What also helps is a proper crew briefing before leaving your seat: Discuss the items you think there are weak points among your FOs. See if their memory items are fresh enough, if they have the correct mind set. If they have read AF447 accident reports. But don't forget all the other risks that are somewhat been forgotten: diversion, emergency descent, fire and smoke.