Now -- this really depends on your interpretation of history. Yes, most of Europe's battles were fought on open fields, but remember. Both sides were fighting on open fields. However, in the French and Indian War, the British showed that they were heavily inadequate in the American terrain, filled much with forest. When the American Revolution and the War of 1812 came along, however, the Americans showed that they could use the forest terrain to their advantage in order to defeat the much larger, more powerful British Army in those open fields. Of course, the tides were turned in the Vietnam War, when the Vietcong ravaged the American Army in the forests.
However, if an army doesn't really know how to use guerrila tactics, and such army just coincidentally was fighting out of a forested terrain against an open terrain, I do rather agree that a -33% penalty wouldn't be fair automatically. Rather -- I would have it so that there should be more bonuses for units that wanted to concentrate on using guerrila tactics, and those bonuses would only apply in forested areas.
One bonus that would be nice would be for a unit with such bonus to be invisible in forested areas.