Those squeaks do show up in guitar Midi. Unfortunately they show up as freak notes because Midi can't recognize them properly. Transients from guitar are a big problem. The rule of 18 and its timbre verses piano notes.
You can actually edit them out in the piano roll veiw a lot easier than trying to remove them in audio.
Deminsion Pro has a very nice Martin Steel String sound in their included files.
ORIGINAL: spindlebox
I just thought of one thing reading this thread. Some MIDI-philes would KILL or pay a lot of money to have those squeaks in their "guitar" tracks.
Having some gives your music that organic feel that so much of todays recorded/electronic music lacks.