If it doesn't re-encode (which it definitely doesn't) there can't be any artifacts. I've carefully compared many Original clips vs same Trimmed clips...absolutely indistinguishable.
I prefer not to mess around with original files in-camera. Transferring originals to the HDD allows frame-accurate trimming on a large monitor...with renaming. A lot easier on the eyes as well...
I should have mentioned that it's only for Windows...not that other stuff.
I'm looking for a good AVCHD trimmer software. The in camera thing works, but it's a bit fiddly. @Skeptikal "Smart Cutter" might do the trick, bur I've read reviews that mentions artifacts. Downloading the trial now to check it out.
Or, even simpler. Use the in camera video divide function, you could use it to just trim the first second of the clip while leaving the rest squeaky clean.