A little over a year ago Nick Ames put together this handy graphic demonstrating his recommendations for designing printable objects with small holes. The reason this is even a concern is that a printer’s hardware takes longer to calculate the print head’s path on objects that are closer to a true circle. ((The RepRap wiki has a lot of math explaining what’s going on here.)) If the calculations are taking too long then the extruder may end up depositing a little bit of extra plastic causing “blobbies.” This is definitely not a big problem and merely requires some extra clean up which is easily accomplished with a drill bit or sanding. I would offer a few caveats to Nick’s recommendations. First, the slower you’re printing the less this is an issue. Second, these recommendations are really only relevant to vertical holes. If you’ve got a hole running in the XY plane, there are other considerations – like overhang issues. For small M3 size holes in the XY plane, I use a high-vertex count polygon or even a true circle. In those instances the holes are so small that overhangs aren’t a big deal. And since the hole is being sliced on its side, the hardware doesn’t have to perform a lot of tough calculations. Third, unless there’s a cosmetic reason go ahead and print with the smallest vertex count polygon your design permits. There probably won’t be a structural difference and you’ll have less clean up post-printing. A special thanks and kudos to Jordan Mills for finding Nick’s post and graphic for me!