To those who think there should be no apostrophe in terms such as b's:
read the following.

Don't put two many as in "as." [unclear]
Don't put two many a's in "as." [clear]

How many is are there in "is?" [unclear]
How many i's are there in "is?" [clear]

As for the matter of double quotation marks versus single, I've recently decided to use singles whenever possible, just because shifting to get the doubles is an extra step. Also, I have such short little fingers that when reaching for the shift key, I often hit something else.

I have noticed that single quotes are often used for dialog in books.