I am kinda a BTTF nerd. I have been graphing them for the 2 years pretty hardcore. Here is a little of the background I can add to who signs.
MJF- he rarely does anything public but when he does he will do them up. I was able to get 3 posters and 2 11x14 pictures done.
Tom Wilson (Biff)- He has grown tired of the whole BTTF thing. Earlier he embraced it and even did some appearances based on BTTF. I first got his autograph about 5 years ago and he didnt have a problem signing anything BTTF related. He even answered mail and sent out pictures. He slowly quit sending out photos and even answering autograph request overall. I saw him at a comedy club last year and he signed 4 BTTF related items for me. He seemed very interested in getting the interaction over with (I didn't mention anything about BTTF, instead talked about other projects he was involved in). He told me as of April 2009 he was taking a vacation from BTTF autographs. He listed on his blog that he wont sign anything BTTF again, ever. They did a Hollywood Show BTTF reunion and he did not attend. He will be the most difficult BTTF autograph to obtain from now on.
Chris Lloyd- Lloyd attended the April Hollywood reunion show and signed for a nominal fee. He has always been a decent autograph. He is pretty quiet and reserved.
Crispin Glover- He willing signs just about everything but yes as another poster said he is out there. I have seen autographs where he signs Crispinglover.com or will leave a phone number for you to get a hold of him. I have personally never got his autograph but heard many unique stories about people who have met him. Does not seem to be receptive to TTM autographs.
Lea Thompson- She is a receptive signer in person and TTM. She attended the BTTF reunion and charged a nominal fee.
Almost every fringe cast member attends reunion and collectible shows.
Claudia Wells (played Marty's GF in the first movie) did not back out of acting in the 2nd or 3rd movie, her mother was very ill and chose not to come back and be apart of the other movies, it was not money based. She did attend the BTTF reunion show.
Crispin Glover was written out because he wanted too much money (MJF money) and final script approval.
Hope this helps out other collectors.