JetService- LOL! I didn't notice that about the letters in W. Springfield!
Continental- We do see an image of Maggie in the future. I don't know what episode, but it takes place in the future, because Marge & Homer have aged somewhat. In the quick scene "Future Maggie" is in, she is tying up the phone line & smoking, wearing a leather jacket & appears to be about 17 years old.
Following the point that Seven_Fifty7 started, in "Beavis & Butt-Head", they lived in Highland, (Daria's hometown before she moved to Lawndale), but the state is never named. I think this is intentional.
As for my input, I think either Massachusetts or California, because it's near the water & has mountains, but we'll probably never know.
A310/319/320/321/333, ARJ, BN2, B722/73S/733/734/735/73G/738/739/744/757/753/767/763/764/777, CR1/2/7/9, DH6, 328, EM2/ERJ/E70/E75/E90, F28/100, J31, L10/12/15, DC9/D93/D94/D95/M80/M88/M90/D10, SF3, SST