It depends on what you are testing. If you want a marker for liver damage, then drawing a GGT during the cycle is the way to go. After the cycle is over your LFT's (Liver Function Tests) will normally return to baseline. You may have done real damage to your liver along the way, though, and would now have a false sense of security about it, not knowing that you may have done significant damage.
This is a lot like the old axe you hear that as long as the Lipid Profile returns to normal after the cycle, you are okay. Well, during the time when it isn't, you are caking the inside of your cardiovascular system with plaque. Returning your blood fats to baseline later doesn't mean you haven't hurt yourself in the long run.