Yeah, on the whole most things can be done without constraining ones self to the HEX except for a few minor things:

Bluffs

&

Hedgerows.

Those two items only seem extremely confined to HEXES as the rules for them specifically mention them running along a HEX edge.

Mostly though, an terrain feature must fill most of the hex for that hex to be considered that terrain. The DOTs in the center of the hexes are used for determining Line Of Sight (you check from centre of one hex to centre of another hex), and if it crosses a hex of LoS blocking terrain, a piece cannot see another piece.