I've always questioned the need for a 3/4" gap all around the floor to allow for wood expansion because:
1. In my experience wood never shrinks along the grain (the length of the board )as much as it does across the width of the board.
2. if the expansion/contraction occurs in each board then the nails holding the boards near the edges are going to rip right through the tongue, and I'd think the boards would try to ride up the nail.
Nonetheless, I cut out the drywall and did provide the gap because I wasn't going to risk a bad floor just to prove a theory!
Other thoughts...
I installed my floor in the winter when everything was at its dryest, and the boards are still as tight as they were 10 years ago. My coworker who installed in the humid summer has gaps in the winter. So it seems to me to better to install in the winter. Of course I never open up my windows and so my humidity is probably well regulated. This might be a huge factor, and maybe it is better suffer some gaps than have it buckle.
The flooring store installer told me that he thought manual nailers provided a better finished product because you swing much harder and force the boards tightly together.
As someone else said, a day of bending over installing flooring leads to a very sore back!
Regards,
Brent