I like shooting at big white diamonds (squares turned 45º) with no mark in the middle. I don't know if it's the right thing to do, but the plain white background helps me see the gap between the front and rear sights. The top and bottom points give me a right/left reference, and the left and right points line up with the top edge of the rear sight. It really helps me shoot good groups at 25 and 50 yards.
But I think it's always good to mix it up.