Lylanthwol said:
Instead of doing this with a Bonehew, it's generally recommended that you socket it with 1 Amn and 1 Shael. This will enable to you to reach the next IAS breakpoint on your Merc, meaning that he will attack faster, which will also mean that he will leach faster. This will increase your attacking speed as well as providing you with all the leach you will ever need. Better still is to socket with 1 Amn and 1 IAS jewel that has another useful mod on it, such as +ED%. You'll still reach the IAS breakpoint, as well as getting the benefit of the other mod, although these are obviously less hard to come by than Shael.
Another popular choice for a Summoner Merc is a Hone Sundan, for the abundance of Crushing Blow that it provides, or a +4 Ariocs for the boost to your might aura. Some people also advocate the Kelpie Snare for the slow attack mod, but I'm not a fan of this as gumbie and Decrep can easily take care of this attribute. My personal choice is the Bonehew.
Crushing Blow is extremely handy however, especially against bosses, so I use a Guillaime's Face for the 35% CB it gives. Combine this with a Duriel's Shell for Resists / CBF and you have a very versatile Merc setup.
The only problem you are likely to encounter will be Oblivion Knights who cast Iron Maiden on your merc, meaning that he invariably kills himself when he attacks. One way of getting round this is to remove the Merc's weapon until the curse wears off, reducing his damage so that only a negligible amount is returned when he attacks. This can be a bit of a pain however, particularly if you have space problems. Therefore if he gets IM'd, I tend to just accept it if he dies, but only Revive him when I have cleared the IM-infested area.