If you can't see wood rot on the tops of the boards, you aren't likely going to find it on the bottom of the boards. Water under the boards is going to drip off before it soaks too deeply into the wood. Wood has to be wet for a long time before it starts to rot, and typically that means that water has to be trapped in a crevice it can't drip out of, or is in contact with wet ground for a long time. So, the bottoms of your porch boards and steps will typically be in better condition than the tops.
If you do replace any boards, I'd be inclined to use pressure treated lumber as the replacement. Be aware that the new ACQ (Alkaline Copper Quaternary) pressure treated lumber needs either triple dipped galvanized fasteners or stainless steel fasteners to stand up well in it. Regular galvanized fasteners will corrode quickly in the new pressure treated lumber because of it's very high copper content. Previously, pressure treated lumber was treated with something called CCA (for Chromated Copper Arsenate), but there has been growing pressure to eliminate the arsenic because of mounting evidence that it was leaching out of the pressure treated wood and into the environment. When they removed the arsenic, they found they had to use 4 times as much copper to provide the same level of protection as CCA. Since galvanized fasteners have a zinc coating and the wood pressure treatment is rich in copper, you have something similar to galvanic corrosion happening every time the wood gets wet. So, we now need to use more corrosion resistant fasteners and hardware to stand up on ACQ pressure treated wood.