As a practical matter, I use what I call "tilted plane" greens, i.e. constant slope in one direction, like tipping a piece of cardboard up at a slight angle on slightly uphill holes where any (even small) roll could tend to blind the putting surface.
I have experimented a bit, and find that putts on these greens can easily get away from golfers, with no counter slopes to stop the roll. The max slope to be about 2.25% for comfortable putting. 2.5% gets a bit dicey on green speeds over 10 for those of us who are average putters. Of course, according to the Jerry Lemons chart or the PGA Tour cup setting guides, you could get close to 4% (with no cross slope at 90 degrees) for those guys, even at speeds of 13.