I wouldn't use decking screws - go to your store and ask or buy trim head screws. These have small heads that minimize the chances of splitting the wood, but you should still drill your pilot holes.
As for mounting the cabinets to the wall, use cabinet screws which have wide washer type heads which pull the cabinets into the wall.
I don't pre assemble the A-B-C cabinets - I install them one at a time. They are less heavy... just my preference. I can still get the nice flush fit
Its' never hot or cold in New Hampshire... its' always seasonal.