Whether you should block these monsters or not fully depends on your goals in OSRS and your playing style. They are a very common task from the higher level slayer masters, so you need to ask yourself what you want to achieve with slayer and how you want to train it. Slayer points to block tasks are valuable, and you can only block a limited amount of tasks, so don't take the decision lightly.  If you are doing monsters for high-value drops, you should only block hellhounds if you are not going to do clues. If you are going to do clues, hellhounds will give you a chance to give you items like third age robe tops. These are worth tens of millions of GP. Hellhounds are a good option to do if you are going for a maximum exp an hour. If you do, you should cannon them and use prayer while in your best in slot melee gear. You will easily get 50 to 70k slayer exp doing this.  If you love playing AFK as I do, then you should kill them in the catacombs of Kourend. You can AFK there for a couple of minutes by using prayer. You also have a pretty easy way to reset your agro by just running past the altar and return.