I'm going to buck the other two answers and tell you
DO IT RIGHT OR DON'T DO IT.
It is very important to torque your head bolts correctly. The actual preferred method for attaining the proper torque is by figuring out the fastener stretch, not by using a torque wrench. When torquing, you are applying a clamping load on the object you are torquing. You can get this at its most accurate by measuring the stretch of the fastener by using a dial indicator. It's by far easier to torque the fastener down using a torque wrench and in most applications, this is an adequate method of doing things.
In your case it seems you are using torque-to-yield (T2Y) fasteners. These are (as @rpmerf pointed out) single use fasteners only (NOTE: If I could make this text blink, I would to emphasize the point). These must be replaced once you have broken them free (this would blink as well). If you don't, your head gasket will fail. The head gasket is the typical place for these types of fasteners. The reason these fasteners are only good for one go around is they are designed to go up to that "fail point", but not beyond. They start to fail by giving a bit or stretching. These are specific built fasteners by the manufacturer. They have done the stretch engineering for you. By applying the last degree turn, you are stretching the bolt to the tolerance they have engineered into the bolt. If you go past this point, you are only courting death for your engine. If you don't go far enough, you'll have a weak point in your torque chain which could allow for warpage in the part you are trying to affix ... especially true for a head.
As rpmerf stated, get the torque angle gauge: