If I only go up to 3 ft from the leaning wall back for my ladder's feet, I start getting pretty steep on the angle it closes approaching 90 degrees. I feel (but I question my intuition) that I am safer when the ladder is leaning less steep against the wall as it feels like more weight is pushing against the supporting wall. If my ladder is extended back only 3' and goes up 25' (approx 82 degree angle), would that be considered unsafe?
The rule of thumb is a 1:3 ratio--1 unit out for every three up. Three feet would only be suitable for a ladder up to 9 feet high. Three feet out for a 25 foot ladder is absurd. You'd topple before hitting the halfway point. It should be at least eight feet out.
So yes, slope matters, and too steep a slope can easily result in an arcing backward ride through space. Of course, you must be sure you have adequate friction or spike grip at the bottom. Less slope means more lateral force on the ladder's feet.