"Hold low when shooting uphill or downhill" is what it sez in my Speer handloader's guide, version 12. What it is, is, the cosine of the angle is your multiplier. In other words, if a target is 300 yards away and uphill (or downhill) at a 30-degree angle, the cosine is 0.866.
.866 X 300 = 259 yards, actual horizontal distance or "effective" distance. For the typical .243/.270/'06, this means about two inches less drop than usual. Four inches instead of six inches.
In other words, if it's no more than 300 yards in distance, and no more than 30 degrees, forget it. If it's on up toward 45 degrees and inside of 300 yards, you might hold a couple of inches lower than usual.
FWIW, Art
[This message has been edited by Art Eatman (edited April 16, 2000).]