Would it have been either of these?
1) Change Max Slope Value (From default 20 to 50 or 89, depending on the one you pick) Though it auto-levels the lot to be reletively flat after placement.
http://www.modthesims2.com/showthread.php?t=47544
2) Lot Placement Hack (Leaves lot on whatever slope/angle it's on.)
http://www.modthesims2.com/showthread.php?t=227576
-ZW