Most frames I encounter at work don't have this done and need reasonable amounts of prep work. The main reason this is needed is because metal frame tubing distorts from the heat applied from brazing or welding, so the bearing surfaces need remachining with special milling and tapping tools that are cheap enough so that a shop doesn't have to buy a vertical mill or thread tapping machine. Prep work doesn't simply remove paint in most cases, it just looks that way. The paint merely acts as a tracker when facing a BB shell or head tube; the paint that remains is just a low spot that can indicate more facing is needed for example.
BB shell threads should ideally be tapped with a piloted tap set (not just chased), so they are aligned on a common axis. The facing procedure then references the aligned threads so that everything is nice and square.
For Shimano-type cartridge BBs, tapping may be needed so that the plastic or aluminum left side cup doesn't get stripped from poor quality threads, but facing is unnecessary. Cartridge bearing BBs that have separate replaceable cartridge bearings in individual cups that reference the shell faces do benefit from tapping/facing, and for external bearing BBs it should be considered a necessity. Many of the external bearing BBs have little tolerance for unfaced shells (Truvativ cranks are the worst in my experience) and the bearings seem to crap out rather quickly if this is overlooked. I usually recommend that customers have this done before installing any external bearing BB.
Surprisingly, I've seen some of the mail-order frames (e.g. Douglas from CC) fully prepped, where as most frames from the bigger companies like Trek, Giant and Specialized usually forego any prep work. Most high-end small builders seem to do this. If you're investing in a nice Chris King headset or an external bearing crankset, it's worth paying a little more to get the frame prepped by a competent shop (you may have to try a few shops; find one you know has good mechanics and tools, a couple I've worked for either had no machining tools or had poorly maintained/abused tools). It only has to be done once, and it's usually cheap insurance for better bearing life and performance. HTH.