There is a book for regional differences. It's Signs Across America, A Look at Regional Differences in American Sign Language, by Edgar H. Shroyer and Susan P. Shroyer (published by Gallaudet College Press, 1984). Even it's not 100 percent accurate but it's interesting.
Not logical? Living languages are never fully "logical". (Hmm; Latin was pretty logical--where is it now?) The differences aren't enough to shut down communication unless you get hung up on the differences instead of just going with the flow.
What if? I can almost guarantee that you as a terp will run into regional differences, and you better be ready to adapt. As Etoile posted, VRS terps get connected to clients all over the country. Community terps work with deaf consumers who move to the local area from all over the country (or from other countries). You WILL see regional signs, and you WILL adapt, or you won't make it in the field.
How's that for "blunt"? 
