There are two types of head bolts. Those that you use once, and torque to yeild bolts.
The 4.2L has the type that you can use many times.
The 4.0L has the torque to yeild bolts.
The torque to yeild bolts will acutally strech when torqued. This applies bolt thread pressure to the cylinder block threads, securing the head. If you attempt to use this type of bolt a second time the stretched bolt often will snap in two. Then you're in for a treat removing the bolt. The stretching also makes re-troquing the bolt impossible because the bolt no longer holds manufacture tollerences.
It's personal oppinion of mine to repace the bolts regardless of the type because you may not be aware of the boltsí history (overtorque, etc.). I don't need a used bolt snapping, binding, double threading, or causing more headaches such as a warped head.
Though I'm cheap, I'm cheap for a reason. This is one time where I defer in the area of extra cost. In the long run when a head bolt problem never occurs it is money well spent.
I always install headbolts with a drop or two of light oil on the threads. It eases insertion. A couple of drops under the head of the bolt also eases contact with the head during torque. Some installers install dry because they feel the oil throws off the torque seting. Though I can agree in principle, I can't agree in practice.
Thighten the bolts to specification by dividing the total torque setting into three steps. Use a criss-cross inside to outside pattern when torquing and you'll end up with a quality job.