When did they stop teaching to file "Mc" before "Ma"? I see the rules now to treat "Mc" the same as "Mac", which would mean that "Ma" would come before "Mc". But when did this change and why?

Who's the "they" you are referring to? I believe it's always been the case that "Mc" is treated as if it was "Mac," and both are alphabetized as "Mac." At least that's what I was taught way back in the 60's. Hence the name "Maas" comes before "McDougal", which comes before "MacKinley," which comes before "Marvin." But I suspect that different organizations have different rules - the phone compamy for example has some interesting rules about alphabetinzing personal names versus corporate names.

Blame the advent of computers --

M', Mc and Mac
Historically, the prefixes Mc and M' in Irish and Scottish surnames were taken to be abbreviations for Mac, and alphabetized as if they were spelled out as Mac in full. Thus McKinley might be listed before Mackintosh (as it would be if it had been spelled out as "MacKinley"). Since the advent of computer-sorted lists, this type of alphabetization is less frequently encountered, though it is still used in British phone books.

Alphabetical order - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Alphabetical_order)

The OP is suggesting that MacKinley ought to come before Maas. I was never taught that.

The OP is suggesting that MacKinley ought to come before Maas. I was never taught that.
I must be older than you. In grade school, I was taught Mac before Mc. Before that Mac/Mc was filed first before Maa-. Now, Mac = Mc and all Macs are filed under Mc.

In Libraryland, we had to periodically re-alphabetize our paper catalog cards as a new edition of the AACR came out.