Is the iSlack an absolute necessity for phone work?

First, if you are repairing an iPhone 4S, an iSclack is not needed. The back glass/panel of the 4S slides of the device after you have removed the two pentalobe screws at the bottom of the iPhone. Second, for other devices that require being opened with more force, an iSclack is not necessary although it is going to make a repair much easier. Definitely, recommend it to make your repair go smoothly. If you don't feel like spending $25, use any suction cup. Of course, those require more force and could damage your device if your not careful, but they're inexpensive.