I think this is an excellent quesion - thanks for asking Travis.
This is simply from my own observations and experiences, but I believe it is more practical to have the boat just in gear and approach from as great a distance that the conditions and your casting ability allows for.
You don't typically 'pepper' one spot - usually a dozen or so casts in the area you are fishing will indicate the prescence of life and whether a huge dominant leviathan will have a stab. By slowly motoring along you still have 'enough' time to get adequate casts into the area and continue on to the next area (I say this on the assumption you are fishing a reef edge/system). You are covering more ground and subsequently increasing the likelihood of encountering more strikes from GTs. If you are fishing a solitary rock or outcrop, I would still have the boat in gear/idle as the skipper may need to take immediate action in order to position the boat, provide the angler with the best fighting position and assist to pull the fish away from structure. Responding in the fist few seconds of th strike could mean the difference between life and death for your expensive popper and morale!!!
I also think this is important as the skipper is able to look out for dangers (bommies, waves, swell), keep the boat in the best possible position/path and also has access to information such as water depth and bottom and act accordingly. At least where I tend to fish, it can also be important to focus on slightly deeper areas - the skipper can take this information into account so that you are fishing the best grounds/path as you motor along the edge/system.
With today's GT rods, thin diameter braided lines and one's casting ability, I do not think spooking fish from 80m+ away is a big problem. I do acknowledge that there may be a decline in strikes, however, this may not always be attributed to the boat - we've all seen G's follow a lure (many fish for that matter) and simply turn away due to lack of interest (maybe it's our technique???)
Just my 2C.
Warwick, GTs definitely do become more timid in clear/calm water. In a nutshell, I would advise the use of a stickbait for these situations - however, we all know that a huge GT may strike at anything at anytime - not exactly a science.