I agree with Benzfan; incremental tightening is important on any flanged connection. If one corner is tightened just a little too much initially, it will cock the water pump slightly because of the o-ring. When you tighten the opposite corner it may cause the flange to "bow" slightly, leaving an area of decreased pressure between the bolts. Under pressure this could cause a leak.
When I replaced my water pump recently, I used a medium coarse (80 grit) 6" long sharpening stone (with some light oil to prevent clogging the stone) to flatten the mounting surface on the block. Not really much chance of warpage here, but it helps to get the crud off and ensures a flat surface if you do it carefully.
I used a new pump, but checked the surface nonetheless to make sure it was flat. Then, I just put 4 tiny dabs of wheel bearing grease in the channel where the o-ring goes to hold it in place. Still, it's important to make sure that once you set the pump and o-ring against the block, try not to move it at all while you run in at least two of the bolts. Screw them in till they just touch. After you get all 4 in, finger tighten them lightly in a cross pattern. If you can, get a torque wrench that reads down to 5 lbs and tighten them in 5 lb increments in the same cross pattern till you get to the 15-17 lb./ft.
Good luck with it.