During a textbook rotate, the closest the tail of a B777 will get to the ground is 3.4' (About 1 Metre).
So it's pretty close even when things are going normally. To be honest, slow rotates are more common and a clearance of 4.2' is more usual.
As far as incorrect load and balance might affect things: Well it's pretty unlikely since there is a clever nosewheel sensor to let you know if the stab trim is in the right place. In the event of incorrect loading, the 'green band' for setting the trim would show an out of trim condition and there would be an associated caution. Pretty hard to get away with anything these days!
Despite all these safety measures, a good tug at rotate should result in a nice tail scrape.
LFW